4 ways you can do it:
hammer a hole using a nail or screw (sometimes dents the carto)
drill a hole (not very good as sometimes it catches the stuffing)
Dremel a hole
Use a 3/8" saddle valve from HD, Lowes or Ace.
Instructions from a Supplier:
"We use 3/32", but we don't think it matters too much on Boge's, what size the holes are or how many you make within reason. You can even dremel a slot instead, if that's easier. We drill 2 holes in Boges, kept 1/8-1/2" above the Delrin cap.
Dual-Coils are alot more sensitive to the size and amount of holes that are used. For Dual-Coils, we use and recommend only 1 - 3/32" hole kept just above the cap level.
As we test, evaluate and eventually sell other cartos, we will keep DIYers advised on what each carto requires to work best.
Key thing is to barely break the surface with the hole in all cases."
Cheers, Vapoor.