Installing a splitter is quite easy in concept, can sometimes get difficult in practice.
Before you had DSL you had your telephone service coming from the telephone company to a NID (Network Interface Device), also known as the Protector or the Demarc (demarcation.)
DSL rides on the same pair of wires as your telephone service -- at higher frequencies. These high frequencies create noise on the telephone, a rather annoying hiss. To stop this, you need to employ low-pass filters between your telephone line and any voice service device (which includes 56K modems, fax-modems, and fax machines as well and telephones and answering machines.)
You can use individual filters at each device, or you may install a whole house filter at or near the NID.
Start by checking that all of the telephones work. Taking a photo or making a drawing of what exists helps in case you need to undo your work. You then lift *all* the existing voice wiring off the NID and tie them down on the voice terminals of the splitter. Then you add a jumper from the NID (where you had lifted the above wired off) to the Network terminals of the splitter. At this point you have added the filter to your phoneline and all of the phones should still work. Go check *all* the phones. Fix any problems before preceding, or undo your work and come back here for help. Digital photos help. Assuming that all of the telephones work you may now tie down the wores going to your DSL jack onto the Data Terminals.
As people write to me of troubles I will expand this page.