To hide your words under a layer of paint, take the stencil with the heart cut out and place it over your card. Then take your third red paint and paint over the words. If you want your words to be easy to read, use a transparent pigment. If you want to hide your words completely, use a thick, opaque paint. (I've used cadmium red medium, which is an opaque pigment but I've spread it fairly thinly so the words are readable if you read closely.)
Paint by placing the brush on the stencil and moving it into the heart; this helps prevent paint getting under the stencil and messing up the heart shape. When you've painted the whole heart, lift the stencil. Depending on how carefully you placed your stencil, you might have a thin white outline on the heart in places. Either reposition the stencil and paint this in, or paint it carefully freehand.
That's your Valentine's card just about painted. Sit back while it dries completely and decide whether you want to add a glaze or not...
