If you have access to MS Paint, or the equivalent, you can use this template pretty easily.
This is not exhaustive, but this is the general size and dimensions of the current comic slots I'm offering for commissions right now.
There is a hard rule of only four vertical 'columns', however if you wanted to combine columns into one larger image you can (you will see an example of such in one of my upcoming comics).
I prefer comic panels to be ~7 total, but am okay with 8 if need be or if it makes sense. Please do not try to cram more than that into one commission, as most of the comic will be border between the panels at that point. If a comic has to be more than 6-8 panels, then consider getting two comic slots. It will look much nicer in the end and will be able to give your idea the breadth it deserves.

There are other possible comic panel shapes, inserts etc, and I am open to them, this is primarily for you to have as a reference to build your own comic template. Most comics will start with the 1st column as an introduction, then the middle two columns will be a combination of the other three column panel patterns, and the last one will be another big one. This looks REALLY nice and I suggest starting with that as your basic story board.

My associate  Charn loves storyboarding these things for some reason, so feel free to ask him for assistance. You are welcome to draw stick figures in there to help illustrate your point, use text descriptions (That aren't too complicated) or use letters/numbers in each panel with a list of panel descriptions underneath. Too many words make my eyes go all glassy though, so try to think of EXACTLY what you are looking for when you set this up.