What bike do you have? I have a 1200 Custom and it was fairly involved because I had to do some wiring in the harness that's attached to the risers. Also the directions that came with it were pretty much useless. But it was all worth the trouble! IMHO
I recently did a tach install on my 2002 883. I will second the opinion that the directions are useless. If you don't have a good knowledge of the wiring harness of your bike expect to drop the extra cash for a service manual.