Hi Roy
Just checked my nearly full tin 
1. Heat part to bright red and put into the powder. Move it around so that it sticks all over the part.
2. Re-heat to bright red (careful, you need tinted goggles as it seems to glow incandescently). Plunge straight into cold clean water.
For a deeper case, do step 1 twice before step 2
(Julian, I've never needed to use water as it has always stuck straight away for me)
Cheers – Will