Wood wall letters are a popular way to decorate nurseries and children's bedrooms. Most hobby or craft stores sell pre-cut wood letters made of MDF board, which is smooth on one side, making it an easy surface to paint on. Use water-based primer and paints which are easy to clean up to keep the project hassle-free.
Sand the edges of the wooden letters with the fine-grit sandpaper until they are smooth. Wipe with a clean, dry rag.
Paint the white acrylic primer coat or gesso using a sponge brush onto the surface and the side edges of the letters. Let this dry and then sand it down with the 220 grit sandpaper. Repeat for a second coat of primer and allow it to dry completely.
Use the sponge brush or roller to paint solid colours with the acrylic paint. Allow to the paint to dry completely between each coat Rinse out your sponge in between applications in the water cup. Do not let acrylic paint dry on brushes or sponges.
Create detailing with small paint brushes. Allow to dry.
Consider using complimentary colours on your letters. Use one solid colour for the background and its compliment for the decoration. Complimentary colours come in pairs and are blue/orange, red/green and yellow/violet. For an alternative to painting, consider using bits of fabric and glue to create a collage onto the letters. If you are looking for a particular motif to match a child's room, there are several websites offering license-free clip art to inspire your painting.
Do not wipe MDF board with a wet rag or the wood will swell and loose it's shape.