Here's what I do...

I put the live mouse in an escape-proof container that has lots of ventilation...and set that cage right beside my snake's enclosure. I also give the snake's enclosure a little bump and shift to sort of wake her up. (This is accomplished by removing the security clips I have on her tub...but any little thump/shift would do)

I leave the mouse nearby for anywhere from 15-30 minutes. This is called "pre-scenting" the room. The snake wakes up and smells the mouse and comes out looking for it...or at least pokes her head out of her hide and waits patiently for it to wander by. You can tell they are awake and searching because their tongue will be flicking quite actively.

Once I'm sure the snake is in "hunting mode," I loosen the lid on her tub, catch the mouse, and drop it in. I try not to drop it right on top of her...but sometimes she will try and come out of the tub and grab it right away! LOL Usually, I try to drop it a little ways away from where she's waiting. And put the lid back quickly!

If a rodent lasts any longer than five minutes inside her tub, I know she's not going to eat it and I take it out. You can wait longer than that because some snakes are slower on the uptake than others....but I would not wait more than 20-30 minutes, tops. If the snake doesn't eat in that amount of time, they can wait another week to try again.

Hope that helps some!