Design AtlasCrop marks sit just outside the actual printed area (bleed included), aligned with where the trim line would extend, as two short strokes — one horizontal, one vertical — kept apart so they never overlap the area that gets cut away.
InDesign and Illustrator generate these automatically when you enable "crop marks" on export. You rarely draw them by hand, but it helps to know they mark the exact trim line and are never meant to end up on the finished piece — they only live in the margin around it.
When several pieces are ganged onto one press sheet, each one needs its own crop marks, which is why print shops ask for files exported with marks intact rather than hand-drawn approximations.
When to use
Before sending a file to a print shop, confirm the export included crop marks. Many online print templates already bake them in, so you often don't draw them yourself.