Wash down the wall on which you want to place the Velcro. The Velcro adheres better to a clean surface.
Allow the wall time to dry. The Velcro is not going to stick properly on the wall when it is wet.
Pull off the protective backing across the back side of the Velcro. The protective strip keeps the adhesive on the Velcro from sticking to unwanted objects.
Press the adhesive side of the Velcro onto the wall and hold it firmly in place. Release your grip and the Velcro will stick to the wall.
Perform the same preparation on the item you want to stick to the wall. Once the other side of the Velcro has been applied to the item, press the item against the Velcro-lined wall. As long as your item is light enough, it will stick to the wall.