This is a standard two-bladed easel. The non-adjustable corner has a paper stop that you can adjust, within limits, to set the desired width of your border. The exact position of the movable blades you need to determine by trial-and-error using a scrap sheet of photo paper or a piece of paper cut down to the size you use. Set the blades, mark the paper and then check border width. Adjust as needed and make marks as suggested above.
For me, I never use a standard size for my images. The aspect ratios are all different, so I just set the longest blade for the paper size I was using and then did the cropping by adjusting enlarger-head height, moving the easel and moving the other blade.
Hope this helps,
Doremus