I found that if you have a small unfiltered tank, you can actually just let it float, but make sure when you put the cup in, you tilt it so it'll let any air bubbles that want be caught on the underside out. You still want to keep an eye on it, though, 'cause **** does happen, but personally I've never had a problem with this method.. Also, if you're acclimating them, make sure you take the cup out of the water before adding the tank water so that it won't become imbalance and you can make sure it stays afloat!
In my 5g, I have a filter, so the water moves enough to where can push the cup and it tends to tip.. I use masking tape so it won't leave any stickies on the tank or lid of the cup, and it's strong enough to hold what it needs to hold. Usually I get a long strand of tape, make sure it's good and snug on the lid without covering the hole with little to no air bubbles under the tape, put the cup in at an angle so there aren't any bubbles underneath the cup, and while holding the cup against and as level with the rim of the tank as possible, I place the rest of the tape on the outside of the tank. It works really well and I do it for both my fish and snail. I've never had problems at all, and I also have a small spoon like thing I keep to the side so I can pour a few spoonfuls down the opening when I'm acclimating them- that way I won't have to take the tape off every time and have the possibility of the tape losing its stickiness nor do I waste tape ^.^ Sorry for the long explanation >.>;;