It is pretty simple. Just route the cable into the frame on the front. It'll push through the TT. On the other side, by the seat mast. You will notice that the gromet is removable with a hex wrench (I think 1.5mm - not sure - at least on my 09 Madone it is metric but on my early 08 it was, ASE!: 3/64" or 1/16", I recall it not being metric @ 1, 1.5 or 2mm though.). In good lighting, wiggle the cable around and catch it either with a small pair of needlenose pliers, or a rubber band inserted into the frame. Don't forget to feed the cable back through the gromet again. Carefully tighten the bolt down again before sliding the housing back on the cable.

Retro Grouch's suggestion of using cable housing back through the TT from the rear works great too and avoids having to remove the gromet. For me though, when I am pulling brake cables, I am almost always stripping the frame too, and never remember to tape off the frame with the housing in place. It is good for an inplace cable change, but I am usually stripping the frame when the brake cables are coming off, so other work on the bare frame before I even think about putting new cables back in.

HTH
zac