Two main things.
First on the outside. The shoulder on the front of the thumb safety that interacts with the plunger may need some adjustment. This will depend on what safety you choose as well as the shape of the plunger tip that it will interface with.
Next on the inside. The sear engagement lug will often need some filing and/or stoning. The way to tell if this is going to need fitting is, if the safety seems mushy on/off instead of having a nice "click" on and off -- hold the hammer all the way to the rear (beyond where it would normally sit when cocked) and disengage the safety. If it seems better with the hammer pressed all the way back then the sear engagement lug needs some attention. It's typical to see a flat surface on the lug where some filing has already been done, but not always. Knowing exactly what the geometry of that lug should be is something that you should learn before attempting it. There are probably some videos on the 'net that explain it. Sorry I don't have any good pictures handy.
In either case, go slowly to avoid removing too much material because that will ruin your safety. It is essential that the thumb safety works properly in order for the gun to be used safely.