By applying gel over a stencil and then flash curing the product before it had a chance to level...
Nail Art Tutorial: The Plastic Wrap Technique Using Gel
Step 7: Dip a piece of plastic (sandwich bag, paraffin bag, or plastic wrap) gently in uncured Mani-Q.
Step 8: Press down making sure not to slide the plastic around too much, which could cause colors to become muddy.
Step 9: Press plastic in to small areas where more or less Mani-Q is desired (your preference).