Sealing natural slate tiles

At what point in the whole process should I seal my natural slate tiles, before I lay them or once laid but before grouting?

It really is a very simple process to do and requires no special skills or preparation to lay a simple slate floor. It is exactly the same as laying a ceramic but the only difference would be that you seal before and after grouting. The whole process would go like this

1) Bond floor/wall
2) Lay slate tiles
3) Clean slate tiles
4) Seal slate tiles
5) Grout slate tiles
6) Clean slate tiles
7) Seal slate tiles
8) Finish

This is basically the same process for any natural stone if it be slate, limestone, etc etc