using a grid!
First, you need to do a grid on RMD. There is no rule so you need do it with your eyes. 1 line in the middle (at 50%) and 2 more (25% and 75%). Same thing the other side. When it seems okay, you save it in your computer.
You need 2 grids. 1 horizontal and 1 vertical. So, when you want to do a drawing, you can load the grid that you need.
Your photo need to be at the same ratio of your RMD page. The RMD page is 460x640 so if you photo is 500x500, you need to calculate 460x500/640. In photoshop, you open a new project with 359x500 pixels.
If you have photoshop, import your photo. Go on «affichage» (view i think) and «règles» (rules). Now, right click on the rule and choose %. To add lines, click left on the rule, let the button clicked and slide the line to 50%. One more time to 75%, etc.
Now, you put your phootshop window juste beside RMD by minimize the window size. You create another layer and you can do your first sketch on it.
If you need help to tittle details, add more lines. But it need to ALWAYS be on the same ratio.