if you haven't already added water, add just a tiny bit, I added about a cup or less instead of the 4 the directions suggest. I then broke up the clumps that remained, dried it out in a 5 gal bucket in the nice warm sun. Sun will take too long though if you added a ton of water to the brick so baking would probably be better.
If you don't add a sufficient amount of water, it it a lot more work mixing with your hands. An alternative is to spend a little more money and but the dry bags, or just go with chemical free potting soil.