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Lay down the number of blinds equal to the width of your stencil. That could be 2 to 4 blinds in a row. Line them up, making sure the ends are even with each other. It can help to tape down the blinds to the surface at the top and bottom using painter's tape. Start at the top of the blinds. Line up the stencil across the blinds, and lay it against the blinds. Secure it with painter's tape, which will come off more easily than regular tape. Use a round stencil brush, and add a tiny bit of paint. You don't want to use too much, as you'll end up with a mess. Dab off any extra paint on a sponge or paper towel.  You can buy stencil creme at craft stores. You can also use other vinyl or fabric paints. Use the brush to lightly dab at the open areas of the stencil, working the paint into the design. Add more paint to your brush as needed.  Depending on your stencil, you may not want to fill it in perfectly from edge to edge. A messier look may be more appropriate. As you finish one section, remove the stencil and lift up the painter's tape. Move the stencil down the blinds to a new position, lining it up with what you just painted. Paint the new section. Continue moving it down until you've painted the set of blinds you have laid out. Lay out your blinds, section by section. Paint each section using this technique until you have the whole set done. Let them dry before hanging them back up. Check the paint to see how long it takes to dry, but you'll likely want to let the blinds dry overnight.

Summary:
Lay the blinds down next to each other. Tape the stencil at the top with painter's tape. Place a small amount of stencil creme on your brush. Dab the stencil with the brush. Move the stencil down. Repeat for all of your blinds.