If you're doing a standard shape visor, you should have no problems. Different shapes will be harder, such as mine - the extra cutout isn't as thick as the normal T cutout areas so unmodded looks a little odd and has large gaps. What I did to get around this was to:
Cut the visor itself pretty close to the exact size of the cutout
Sand down the higher spots on the inside
Use some filler on the inside
And this is the big thing: Painted the inside edge of the visor matte black. Any remaining gap (And there was some, and as has been said that's a good thing) now doesn't show up half as bad as before because of that one little detail
I've got two Asok CC helmets sitting near me just now, they're excellent. You've certainly made a good purchase.