the vet showed me a "hold" basically, if you're having to do it by yourself, but to be honest, it "sounds" (and looks...when they do it...) easier than it is if you're concerned about holding them too hard or anything like that....
basically you hold the the cham with its side against your palm and gently grip with your forefinger and thumb directly behind the lip line (the jaw)....if you apply easy pressure on the jaws, it will open its mouth....
the vet also gave me a "tip" for the end of the syringe, if you're not needing to "inject" anything, just syringe into the mouth, that the cham can bite down on....it keeps an opening open while the cham is biting so you can plunge a liquid into the mouth... don't know if you could ask for one if they didn't give it to you...it's just a rubber piece....
hope this helps...not sure if that was a clear enough description for you of the grip... but i tried! lol.