There are several how-to videos on setting a mousetrap on Youtube. The majority of the how-to videos detail setting up a traditional Victor mousetrap, but some go over how to set up a door mousetrap.
Set up a traditional Victor mousetrap by first determining where the mice are generally located. Then, open the mousetrap packaging, taking care to remove the staple holding the snap wire to the wood stand. Next, choose a bait, and place it on the mousetrap trigger so it covers the trigger farthest from the spring. Pull back the snap wire underneath the lock bar, and stick the lock bar underneath the trigger hook.