Here comes the fun part... my kids joyously referred to this particular chore as "let's whack Daddy!" They volunteer for it every time and quite frankly, it's got me rather nervous, but it's got to be done so get whackn!
For this chore I use an old pair of blue jeans, (I know you were saving a pair for something like this). Just load up a leg with a manageable amount. You'll figure out how much is enough pretty quick. Special note, if you pack it too full your crotch will pop... really messy.
Now grab your trusty small hand sledge and just lightly pound the hell out of it. Like I said, my kids think this is hilarious.
At this point I feel I should point out another advantage of using crushed clay to make your mix... it's a lot easier to get an accurate weight ratio of clay to colorant you'll need.
I like strong colors, so I multiply the weight of my crushed clay by 5% to 10% to get the weight needed for my oxide colorant. Commercial stains seem to like the 10% to 15% ranges. It's really up to you how strong you like your colors. This is where you get out those test tiles and experiment.
Slowly add your crushed clay and oxide mix to the water. Keep at it, slowly building up an island in the center. The island will subside below the waterline as it pulls water into the mix. Keep adding your crushed clay mix until it all in there. Kind of give the tub a small shake to collapse the island one last time, and let the mix slake overnight. In the morning, just pour off the excess water and stir well and "Voila!" A simple slip.