This is a mimic of a much more expensive tool sold by Park tools to remove headset cups out of bicycle frames. Theirs is all pretty and made of high tensile steel, but whatever.

Find some tubing. Copper, steel, even an old flat handlebar will work. Just don't use aluminum; it's too brittle.

Step 1: Cut Your Tubing Down the Center

Cut down about 5 or 6 inches through both sides. A hack saw will work too.

Step 2: Stretch Apart

Clamp your tube into a vise and using something, separate the ends.

Step 3: Make It Look Like This

It should be about 1.5" wide

Step 4: Remove a Cup

If you wish to remove the bottom cup, insert your tool (non-cut side first). Pull on the top until your flared out bottom rises just above the cup you're removing and hear it pop.

Step 5: Wail

Now, holding the frame securely, wail down on it. Bigger harder hits are better. If you tap tap tap on it, you'll get nowhere other than harming your newly crafted tool.