Shift to the small ring in the front and the big ring in the back (so you have very little tension on the front cable). Use your allen wrench to loosen the bolt, I use a pair of pliers to pull the cable ever so slightly tighter and then tighten the bolt down.
I'm no "wrench", and I might get a lot of flack, but this has worked for me.

Still, I would take it to the shop and let them make the adjustments, and then read up so in the future you can make them yourself.

Good luck!