Pay the shop to do it

Really there are a number of tricks. Some bikes have removable port covers or Bb guides and the cable can be played with far better then if only a tiny hole is present at each end of the routing. Some bikes come with an internal tube, like a thin liner, running inside the frame. If so then retaining this liner will serve as a guide to thread the inner cable. I've made tiny hooks and loops out of spokes to help snag and pull an inner cable through a removed port cover. Slightly bending the last 1/2" (or more) of the inner cable lets it flop about and be able to be placed about the frame's insides by twisting the exposed inner cable while trying to slide it in and out a bit. Some have used magnetics if the cable is attractive to them. Some people have used a vacuum to pull a thin thread along the frame's insides and out the hole the vacuum is attached to. Then tying this thread to the inner cable and pulling it through. I'm sure there are other methods but these are the biggies. Andy