Gather your supplies.

Sketch the 'eyes nose and mouth' with a pencil, it doesn't have to look perfect, because you can cut it a bit different.

Now cut it with a stanley knife. box cutting knife, you can also use scissors.

Now you can put the stencil on the shirt, put a plastic bag in the shirt to make sure the paint won't go on the inside of the shirt. You can also use some plastic bags to make sure the ground you're spraying the tee on doesn't get painted too XD
You can use tape to make sure the stencil stays on it's place.

When you've sprayed it, make sur eit dries for a nice long time :)