How do you print on LEGO clone helmets?
This question is a bit confusing. It mentions water slide decals, but then also asks about printing directly on the helmet. 

Water slide decals are what their name implies; decals that can be slid on the item with the aid of water. It's not printing. They are basically very thin stickers. 

Because water slide decals are so thin, they can easily mold onto non-flat surfaces. If they are larger, and the surface is very round or complex, you can score the decal with a hobby knife before applying it to eliminate any wrinkles. 

But in general, yes, water slide decals can be used on LEGO pieces. The decals should come with instructions on how to apply them. You basically make them a little bit wet, and slide them onto the item from the special paper they are sitting on, then remove the protecting film from the top. 

If the piece is going to be handled a lot, you should cover the decal with a clearcoat to protect it. Water slide decal supplies like clearcoat and application tools should be available at any hobby store.