We did this as one of our advent activities. We’ve made a few of these now and they always turn out well.
You need a cheap frame with a card mount. I got these from Poundstretcher. Take the mount out and paint with water colour paints.
I’ve found water colours are great now Eli’s a vaguely competent painter, there’s much less clean up, they dry quickly and he can use them with colouring books as well as free hand. I haven’t bought him any of his own yet so I have to be brave and let him use mine!