Drill guides that attach to the shaft of a portable electric drill sometimes wobble when you're trying to drill a hole into the end or edge of a thin board. Here's a quick solution.

With a handscrew clamp, secure a flat, square piece of scrap stock flush with the edge or end of your workpiece as shown below. Position the clamp flush with the top of the workpiece and the scrap stock. The clamp and scrap stock increase the surface area to stabilize the drill guide, and that enables you to drill straight holes with confidence.
—Clement S. Barlow, Brielle, N.J.