Find an image of a willow tree to use as the stencil pattern. Sources of images include the Internet, books, magazines, personal photos or drawings. Print or make a copy of the image.
Place a piece of acetate over the source image. Tape the acetate and the image together with masking tape. Put the taped acetate and image on a surface suitable for cutting, such as a cutting mat or a smooth board, with the acetate on top.
Cut out the shapes of the willow tree, using a craft knife. Look for the large connecting shapes formed by bunches of foliage, the branches and the trunk. Make sure the shapes emphasize the drooping quality of the willow tree.
Lift up the cut-out shapes, using the blade of the craft knife, and discard them. What should remain is the space around the tree and the empty spaces within the tree -- the places between branches and bunches of foliage where you can see through the tree.
Remove the tape to free the acetate from the source image. Carefully clean up any frays or rough-cut edges with the craft knife.
Spray one side of the willow stencil with an even coat of repositional mounting adhesive to have a tacky-sided stencil.
Mark the stencil location on the desired surface. Measure from all directions to guarantee accuracy and evenness. Stick the stencil to the surface and roll the stencil with a rolling pin or brayer to flatten it. Mask the surrounding area with masking paper, if using spray paint.
Paint over the stencil, letting the paint glide over the whole stencil. Apply paint by brush or with an aerosol can. Make sure the paint is filling all of the empty stencil spaces, but don't apply so much paint that it is dripping, as it may seep behind the stencil and compromise the sharp edges.
Wipe the paint from the stencil when finished, using a sponge or cloth dampened with warm water and dish soap, if using water-based paint, or with mineral spirits, if using oil-based paint.
Store the stencil between whole sheets of acetate.