1. Get a ruler.
2. Get a triangle. A triangle is like a ruler, only it’s shaped like a triangle. It costs like a couple of dollars.
3. Draw straight lines for your panel borders. Use right angles for the corners. That’s what a ruler and triangle can do for you.
4. Measure a distance between your first two panels and then keep using the exact same distance between all your panels in that comic. Those spaces between panels are called gutters. Inconsistently sized gutters look bad and amateurish. Consistently sized gutters look better. It’s a super easy thing to make your comics look way cleaner and more professional. After you try it a few times you don’t have to measure anymore but you DO have to measure at first. So try it!!
EDIT TO ADD: straight lines and consistent gutters is like the comics layout equivalent of figure drawing / anatomy — get the basics down solid before you start getting crazy and stylized. Log some hours doing the most straightforward approach before you decide you’re a genius and can do whatever the hell you want.