Step 1: What you'll need
Type I silicon caulk
Corn starch or 'Talc'
Step 2: Mix the oogoo
I picked up some sample-sized cans of acrylic paint from Lowes (8oz) for a couple of dollars - that size will be more than you'll ever use for Oogoo projects. Dispense enough caulk into a container (or plastic bag) and add a couple of tablespoons of Corn starch. Talc also works because real talc is no longer sold due to toxicity problems - all talc on the market now is merely perfumed corn starch in a more expensive package. Add a few drops of paint for colouring and mix well. The colouring actually helps you know when the corn starch is well mixed in because when the colouring is even throughout, so is the starch.
Step 3: Shape the plug
Once the Oogoo is semi-cured enough to roll into a ball without sticking to your hands, push the ball into your bath or sink and flatten it down. You *could* insert something at this point to make a handle to pull the plug out when you're using it, but it's not really necessary.
Step 4: You're done!
Before the plug is fully cured you *may* want to mark the top with some sort of indentation so that when you use it, it's aligned the same was as it was when it was formed, just in case your plug hole is not perfectly circular - this'll give you an even better fit.
Now have a seat for 15 minutes (if you're Oogoo was mixed properly) and remove the plug. It should have firmed up into a stiff bouncy rubber, and be ready to use!