#1 tip is this - The clay is everything. It may take 100 hamons to dial in your clay application.
#2 tip - Start with much less clay than you think you need. Using the right steel, temperatures, and quenching techniques you can get a nice hamon even with no clay.
#3 tip - Practice, practice, practice. If a hamon runs off the edge or didn't come out right, just re-do it. Move your clay or change the amount of clay as needed to get the hamon to fall where you want.
#4 tip - Start simple. No one is going to come even close to "Willows in front of Fugi" ... so don't try. Start with a simple suguha hamon. When those fall exactly where you want every time, try chogi, gunomi, and others.
I haven't used mine yet, but bought them for my next hamon tasks. From what I read, the Nuclayer system of two clays is really good. The jars are large and should last for 100s of hamons.
I really like them being in two colors so you know the exact placement of the clay line and ashi.