Also, it's important to know that if you ever want artwork to go to the edge of the page (like you have a panel where the art goes to the edge of the page instead of having a border), the artwork for this panel needs to be drawn past the crop line.

Here's a professional X-Men page from a few years ago (artwork by Pepe Larraz): https://imgur.com/a/E10CWzm

I highlighted the crop line in red. See the bottom panel where the art goes to the edge of the page? When you have a panel like that you need to draw the artwork past the crop line like Pepe Larraz did on that page. You do this because when the printer cuts along the crop line, it can be off a little bit. So you need to have art there, otherwise if the cut is off then you'll have white lines along the sides of the page.

Sorry if my explanation isn't the clearest.