Just done this, the parts that catch the slimline handguard on the G3 are mainly all the support sections, you start loping chunks off that and you could weaken the barrel section.
Much easier to get a dremmel or something and cut a few grooves and sections out of the hand guard. These cuts are invisible when assembled since there all done on the inside of the hand grip.
Cant really give you a guide, its a bit trial and error. Cut a bit, try to put on, mark where more cuts are required ect...