The majority of the time I just use a cupped hand and my voicebox, though times do call for a megaphone device to throw the sound better and for that I use my home made birchbarck call. Which also doubles as a visual aide for pulling a bull a little closer, as does an open bare hand if the shoulder blade isn't an option.

The best way is get either videos or audio tapes and mimic the calls. They aren't extremely difficult nor as complex talk like Elk. The best wayI can describe the calls in type is:
Bulls - OWWaaHH
Cows - Like a beef cow except instead of the Moo - ERRR at the beginning. ERRRRRaaaaaahuH! Very Nasal, whiny or pleading!

Technique varies greatly between moose callers sothe more you learn andpractice the better you'll beprepared when you may be on the edge of such an event or trying to coax a bull out. IME bigger bulls get wary as the rut progress they becomewary and tougher to close. The young, dumb and full of ____, are a lot easier to string along and finish.