Mark your design on the rug with the masking tape. Lines are the easiest to achieve. Just remember, the masking tape will be blank space. In other words, wherever the masking tape lies, the paint will NOT be underneath it.
When you have finished taping the entire rug, and the tape is nice and flat, it is time to paint. (Again, make sure you are on a paint-friendly surface. The last thing you want is for your paint to drop onto your nice hardwood floors).