For this design I used a fine tip on the slip trail bottle and relatively thin slip. The slip is made from the same stoneware clay as the tray. After mixing up the slip (just water and bone dry clay scraps) I sieved it through a #60 mesh sieve to remove any larger bits of grog which might have plugged up my slip trail bottle. The tray was soft leather hard when I added the slip trail design. You don't want to wait until the tray is too dry or the slip won't adhere well.