Do you mean how I make my margins? I saved myself a template with the margins on it and always use that for my pages! It was just a box that was made in Photoshop, I don’t remember how it was made (it was 2 yrs ago), but you can just uses the rulers as guides.
Or if you do it in SAI, just make a new canvas at the size you want + margins. For example, if you want your comic to be 8.5x11”, make a canvas at 8.75x11.25”, floodfill it black, then make an 8.5x11” canvas, floodfill it white, and paste it in the black canvas. Kind of a weird roundabout way but…it works if you’re bad at measuring… (I hope that helps?? ;u;)