I made some valentines from a sheet of thickish clear styrene, and it was harder than I thought it would be. I used spraypaint, and x-acto knife, scissors, deco tape, and a fine tipped sharpie.
The styrene has a layer of plastic protecting it, so I cut away the parts I wanted to spray paint white. This was the hard step. The part with plastic on will stay clear, and the part without will be white. My design had a lot of tiny scallops and the plastic film was clingy but not adhesive, and couldn't see the tiny clear pieces.
Then I spray painted the styrene with two coats of white spray paint. This picture was between coats.
When I peeled off the plastic, the cards looked great. They were impossible to cut apart with the x-acto (it only scored the styrene), so I used scissors, which was kind of hard since I didn't want to snap the plastic by bending it too far.
I was going to do a second layer of spray paint, in red, but the cans were in a cage at blick and no one opened it for me when I lurked near it for a while. So, I put lacy red deco tape from daiso on them, as an accent.
I addressed them on the non spray painted side, and wrote little love notes on the back of the stamps since they'll show through.