Warning -- Long read... (gives a flavor of the job and options)
Where and how greatly depend on your wiring as it is today. In most homes, or I should say, on most homes there is a utility service panel that would have been put in by some provider and usually all wires lead back to that box, in older homes it is very common to find an additional terminal block just in side the house also, in which case all wires lead to the inside terminal block and it is wired to the utility box, but the bottom line, you need to see where the existing carrier tapped in at and how your lines are wired now.
Residential home wiring, especially in older homes, have only two wires connected, but have 4 wires in the cable (the other 2 are usually just cut back or wrapped around the ends for future use.)
Next, depending on the number of phone jacks, when they got put in, and the size of the house, you can be on one daisy-chain, or have 2 or more cables leading back to either the terminal block or utility box.
A daisy-chain means you only have one phone cable from inside the house to the utility panel and that from the first wall jack to the last, the wires just go jack-to-jack inside the house and are all on one pair of the wires. If you are wired this way you can tell pretty quickly becasue at the utility box, there will be only one cable leading into your house, or inside the house there is only one cable hooked to the terminal block that leads to jacks in the house than that pair are tied to the pair coming in the house. I will hit wiring to daisy-chain later in this.
More common in newer homes are 2 or more (4 wire) cables run from the house back to the utility box or terminal block in the house. They are then all hooked to the same place in the box. Also in newer home they often use Cat 5 cable, that can be a little trickier.
The first thing you need to decide is how you want to be hooked up, either have some jacks dedicated to one service or the other, or wire the jack for two lines, and finally, you can have 2 lines hooked single line style to a two jack wall plate. In thae latter case, you just plug in any one line phone to the jack you want. (Service A on one Jack and Service B on the other.)
If you took the separate jack approach, then regular 1 line phones would be plugged in to the wall with a normal phone to wall wire. Some jacks will lead to one service and some to the other.
Finally, there is the 2 line approach, in this case you need a phone that is 2 line equiped (cheap) and a 2 line phone cord, many phone cords only have connections for one line hooked up at the connector, you need phone cords with all 4 wires, again no big deal.
Ok, now that you have determined how you are wired you can decide how to go forward.
If you are daisey chained, then that will be the hardest to deal with unless you only need one jack, if that is the case, if possible have the phone modem installed in that area, and just plug the phone in to it. Otherise you need to break the chain, and where you break it is where you tie in the other service, then all the jacks before the break will be one service and all the lines after the break will be the other service.
Approach -- Multi-Line or Single Line per phone jack.
If you want a 2 line phone that is plugged in to one jack, then that phone will need all 4 wires, the 2 in use by the current provider, and the other 2 you can use for the second carrier. To do that you need to find the cable for that line and attach the 2 unused wires to your new service, then go to the wall jack side, and also punch-down or attach the same two wires to the open 2 spots on the jack, very rare but if the jack does not have 4 contacts, then you need to tool down to Home Depot and get new ones for those locations (very cheap).
Pros: Each jack can now be used for either service or both. One line phones will only hook to whatever service is on the standard pair (the two center contacts on the jack/plug), no need to break the daisy chain, just hook to the unused pair of wires and use them.
Cons: Need 2 line phone and cord to get two lines on the same physical phone, you have to wire the jack side and service side correctly. (Still not hard)
Approach -- Separate Jacks for Separate Lines, different locations.
In this case, if it is a daisy-chain, you break the chain at one of the jacks, then the phones before the break on on the current service and those after it would be on the new service, and where you broke the chain is where you tie in the new service. (you can also go wall jack to wall jack and connect the unused wires together to keep the chain intact and just use the other two wires for the new service.)
Pro's: Easiest to wire generally, not confusion over wires.
Con's: Can't have both
Bottom line on the lines, each phone needs two wires, almost all phone cable has 4 wires with only 2 in use, newer homes may find 6 or 8 wires in the cable, but still only 2 will be used.
Pro's: Easiest to wire generally, no confusion over wires.
Con's: Can't have both lines on one phone/use a two line phone.
Now there are still a lot of varitations, and while it may sound complicated it is not, always remember, each phone service needs 2 wires going from it to a wall jack. Say you went the two line approack but have only single line phones and want to keep it tht way, you have a couple of options, you can get a cord that goes from the plate to phone that swaps the outside pair for the inside pair, that would let a one line phone use the other service on that jack. You can put in dual jack face plates, one wired to each service and so on.
If you have never done it I would get a cheap book, phone wiring for newbies (dummy's series, no insult intended), or surf around a bit and there is endless diagrams and info on wiring phones, but a book with big pictures will generally put all the info you need in one spot.
Next tip, Get a phone wiring kit, again [pretty cheap from places like Home Depot, Radio Shack, Best Buy...) What this does is you plug one end of the tester in to a wall jack, then go to the utility box with the other part of the kit and it will make a tone or light up a light (I prefer the tone kind) when you find the pair that ware coming from that jack, you can also see what jackes are tied together if you diconnect and separate all wires at the terminal block.
The tester will also tell you is you have Tip & Ring wired correctly, not that it makes a lot of difference these days, but best to keep it wired right.
Tip#2 -- Get a phone jack to alligator clip connector, again available at most hardware stores and Best Buy type places. This way you can take a phone to the terminal block or utility box and clip on the find the active line.
Tip#3 -- Have a phone for this work that does not require power except through the phone cord. (old fashioned phone, again cheap)
Lessons Learned Tip: Never take anything for granted, say you find at a wall place the green and yellow wires hooked up are hot, you could find at the utility box or terminal block, red and black, thast means somewhere in the chain, someone switched wires... The test set will tell you.
Final Tip, if you can get away with it, make it the service providers issue, tell them what jacks you want the new service on and how. If you have a fairly easy set up they will just deal with it, or they may charge you a little more to deal with it.
I have two services as my employer provides on for work use, the cable company modem and ATT land line tie in to a terminal block inside the house (I have an old house) Comcast put the phone modem on the wall by the terminal block and pulled a pair from my home office and added to that, in my office, they hooked it one line style to dual jack face plate they switchout out for the one I had. I then left the bottom one dedicated to my service, and the top just I re-wired to dual line and got a two line phone, line 1 is Comcast (my line) and line 2 is ATT, the company line.
The advantage of this it I get more functionality, like the ability to conference callers across the two lines, and just one phone in my office. I got a dual line Uniden phone that even lets you have separate voice mail recordings for each line.
Here are some starter links for you:
http://wire-your-phones.com/ <--- Nice pictures covers old to new cable styles and conventions.
http://www.ask-the-electrician.com/telephone-wiring-diagram-1.html <--- Nice pictures of wall plates and wiring, actual photo's.
If you google "phone wiring" or words to that effect there literally is no shortage of hits, and then you can find the one most suited to your skills, knowledge, and tools.
PS -- It really is not hard, just seems a little confusing because of the the potential ways you might be wired now, as soon as you figure that out, the job is not complex at all, after all we are only talking 2 wires per service.
PPS -- The easiest, if this works for you is just having the second line at one location, just tell the installers where you want that one line and they will figure it out. If I recall, I got one location for free as part of the service, meaning, they had to do whatever was needed to give me the service in one location in my home.