1. Martha recommends using a #103 tip. I think you can easily use a #102 or a #104 as well, and I did end up using a #102 for this cake.
2. Make sure the SMALL end of the tip is facing out and the larger end of the tip is touching the cake (slightly) as you pipe. The technique itself is simple, just choose how wide you want your ruffle, then pipe back and forth… going left to right all the way up the side of the cake. I would take a step back from the cake every three or four rows, just to make sure your rows are straight!
3. You can also make straight lines up the side of the crumb coat to help you keep upright all the way around your cake.