This is probably the easiest method to waterproof a cheap tent (such as a Walmart tent or one thats, you know, under 60 or 70 dollars):
Step 1. Mix a tiny bit of the silicone with a little bit of mineral spirits. (Do this in a well ventilated area!)
Step 2. Mix for a long time till the silicone breaks down.
Step 3. It will be a thin enough that you can brush it on, but still a bit thick.
Step 4. Cover the seams a couple times. Also get the seams, the edge of the zippers, where your tie-downs are sewn in, etc. Seal it all!
Step 5. Let it dry for at least 24 hours.