There is some fitting that has to be done in critical areas, such as barrel lug, frame to slide, hammer sear, grip safety. As far as just puting the parts together it is really very simple, there are some videos on youtube that are excellent. But those are guns that are alreadly "fitted". Unless you are a gunsmith or have some experience I would leave the fitting to the professionals. I have a 70series Colt Government Model and with just a few simple tools I can take it completly apart in just a few minutes.