Make a diamond pattern on the wall with masking tape, or attach a diamond-pattern stencil to the wall with tape.
Attach a stirring paddle to a drill and use it to stir the drywall mud until it is smooth. If the bucket is full, remove a cup of drywall mud before you begin stirring to make room for the paddle.
Transfer about 2 cups of drywall mud to the flat top of a drywall hawk. A drywall hawk is a flat board with a handle that makes it easy to scoop up small amounts of plaster. Apply a 1/8-inch-thick layer of plaster to a scrap of plywood with a putty knife or drywall trowel. Move the knife or trowel at an angle through the layer of plaster to create a pattern. Experiment until you are pleased with the effect.
Use a putty knife or drywall trowel to apply the drywall mud to the bare sections of wall inside the tape or stencil. Apply and texture the plaster as you did on the practice plywood.
Peel off the tape or stencil by grasping the top left corner and pulling it toward the bottom right corner. Remove the stencil or tape before the mud dries.
Paint the wall with latex paint and a paint roller, if desired.