Assuming the other person already has an account (if not you need to set him/her up with one first):
1. Go to the start screen (either hit the Windows key on your keyboard or do it from the Charms bar if you have a touch screen).
2. Click on your user account picture in the top right corner of the screen.
3. In the resulting drop-down menu, select the name of the person who wants to sign in and use your computer (or tablet).
4. When that person is done, they should repeat steps 1 and 2, except he should choose “sign out”.
5. Enter your password and Windows will take you back to what you were doing before you used fast windows switching.