Draw the design you want on the card if you are confident you can do it freehand. If not, search for stencil images online at a site such as Spray Paint Stencils. Download the images you want to use and resize them using Microsoft Paint to the appropriate size. If your printer can print directly to card stock, then print it onto the card. If not, print the design onto paper and stick it to the card. If you are drawing, remember that the part you are cutting out forms the shape of the image of what you will make, not the other way around. Remember also that everything needs to stay attached together, without any "floating bits." If you draw an "O," for example, then when you cut it out the middle will fall out. Leave a bit joined to the main body of the stencil.
Cut out the shape of the card with the scalpel or box cutter. If you want to repeat a pattern on the same stencil, use the piece you cut out to draw around. Otherwise, you will not have good uniformity between the shapes.
Stick the stencil onto the wall you want to decorate using masking tape. If you are going to make a repeating pattern that goes across or up the wall it is a good idea to mark out a line very gently in pencil or with masking tape, so that you can align the stencil. Use the spirit level to keep the lines straight.
Open doors or windows for ventilation before you start spraying. Shake the spray can vigorously, put on the mask and goggles and spray paint over the stencil from about 10 inches away, making quick but steady up-and-down and side-to-side movements until the surface is covered. Move the can closer to get definition along the edges of the stencil.
Allow the pattern to dry for about 15 minutes. Move away from the painting site if possible to avoid inhaling fumes. Apply another coat if needed.
Peel the stencil away from the wall. If making a repeating pattern you might want to mark the edge of the stencil first so you can line it up again when you reattach it.