I’ve made many frames for my wife’s paintings. Here’s one for a 20″ x 24″ abstract she recently finished.
It was pretty simple. I cut a 5/8″ dado into some 3/4′ x 2-1/2 primed stock on my router table. Then I used the router and a chisel to cut out the areas need to interlock it all together. I used a little glue and some 1-1/4″ brads after squaring it up.