Take a plastic soda bottle and rinse it out thoroughly. Cut the top part off, like you're making a funnel, then turn it around and re-tape it onto the bottle so that the end where the cap would go is now inside. Knowing what kind of snake it is, toss in something it likes to eat (perhaps a frozen/thawed pinky mouse, or some freeze-dried crickets.) (the term "garden snake" is pretty all-encompassing so I'm not sure specifically what kind of snake you mean)
Set the bottle trap around where you saw the snake last, you'll probably have better luck around the outside edges of the room. Then just leave it alone. Check it once or twice a day and change out the bait regularly.
I've been keeping corn snakes for years as pets and they are very Houdini-like in their ability to escape. A bottle trap is usually the first method anyone suggests for catching an escapee. They get in, eat the bait, then either can't figure their way out again or are too full and lazy to try.