Hi, I'm a graphic designer who sends artwork to the printer every day of the week.. and I've been in the game for 15 years. I'm not sure what you need to know but.. here are a few tips:
1) if you want a fold, then you mark this as a dotted line 'off the page' i.e. as where the bleed off is. So, for example, if it's an A4 DL leaflet (landscape A4 with 2 folds, so it's effectively divided into 3 panels), the you put a dotted line next (but off the page).to where you want the fold to be.
2) A solid line ('tick mark' ,'crop mark') means it will be cut along that line.
3) I tend to leave no less than 3-4mm bleed off.. just in case. Some printing companies can be a bit hit and miss with their trimming, so it doesn't hurt to add a few millimetres more on your bleed.
4) What software are you using, it would help me to supply more info.
I probably haven't answered your question.. but post again and I'll do my best to help.
All the best.