You do just a thin wipe before engraving. Just put a little bit on the rag, quick swipe and done. It just barely coats the top surface making it a little darker and giving more contrast when you engrave it. If you put it on afterwards it kind of defeats the purpose and would darken your engraving as well.
**Edited to add-- It only takes a very small dab of oil. You don’t want to saturate the slate, just give a light coating. Rub it in well, then wipe off any excess. The tile should be darker, but not slick with oil. The oil will continue to absorb and won’t be greasy in a day or so.