There are a couple of videos on YouTube I found one that I hope helps...
It works best if the blinds are smooth, if they are just put the blind on a table and stencil away.
Take down your blinds. Lay them on a table. Close the blinds so they lay flat. Place stencil onto the blinds. Tape the edges just enough to make them stay put. Stencil with craft paint. Remove stencil & replace in another spot. Repeat until you're done. Let them dry overnight. Take outside & lay flat on a flat surface. With clear spray paint, spray the whole thing to seal it. Let it dry completely. Take a utility knife & gently make sure all the blinds are separated. Rehang on the window.