In order to correctly perform this cheat, you must have 1. A tower equipped and 2. An un-equipped part in your inventory. First, click the 'Sell Parts' tab. Click on a part to sell. DO NOT SELL THE PART. Instead, click on the 'Paintshop' tab. slowly move your cursor over the top-right corner of where you paint your tower. You should see a hand. Click on the button there as many times as you want, for as much money you want. :D