Step 10: I've been framed!
After you've got your holes marked, get a centering punch (makes little dents in otherwise smooth finishes that drill bits can center themselves in), and punch in the center of the marks. I found that I got much better results if I did this while the brace or bracket or whatever was still in place, since they don't like holding on too well. (The braces have rounded corners, whereas the frame's got more squared-off corners).
Then, get a couplea high-grip clamps, and a piece of scrap wood that is AT LEAST A LITTLE THICKER THAN THE FRAME. I capitalized that cos it's important. Put the frame on the wood so that the area you're gonna be drilling is on the wood, and clamp both frame and wood down onto the table that you're drilling on top of. Use a drill bit that is equal in size to the hole in the bracket that's already there, and have at it! I found it immensely helpful when drilling the corner holes to use a nut and bolt (nut on the outside) to actually hold one of the braces in place, to help with the positioning of the other 3 holes. They don't have to be perfectly centered, but the closer to it that you can get, the better.
Here's how my setup looked during the side-hole-cutting-outing: