I just saw a special on Hollywood prop guns on Guns & Ammo TV or some channel like that.
In short, it depends on what gun you are trying to convert. The expert guy said that some are hard, some are easy...but they had to do things like use a special barrels, recoil springs, filed down locking lugs (1911), modified magazines and feed ramps.
My suggestion would be to buy a gun already converted for use with blanks. Doing it yourself is not impossible, but it may take more work than you were expecting.