Tape the washi around the front and back of the envelope starting at the bottom and working your way
Continue taping with your desired colors and pattern until the outside of the envelope is covered. It's OK to tape over the Velcro closures.
Grab your Xacto knife and cut around any Velcro openings you may have taped over.
Paint a layer of craft sealer over the washi tape in order to prevent it from peeling up. Let dry.
To add a decorative touch, sew a button onto the top cover. You may need to pierce through the Velcro to do this.