I would think that it would make the most sense to:
1) frame the image area on your adjustable easel;
2) cut a mask equal to the image area;
3) position the mask on the photographic paper using the easel blades as the guide;
4) lift the blades while ensuring the mask stays in place;
5) expose the borders;
6) remove the mask;
7) lower the blades and then adjust them slightly to increase the image area;
8) expose the image, which should bleed slightly into the black borders.
It might help if you could put something tacky on the back of the photographic paper, to ensure it stays still. Some easels may be unsuitable for this, as they may have a tendency to move the paper when you move the blades away.
Actually cutting the mask may be finicky. A mat cutter might be helpful.
1) frame the image area on your adjustable easel;
2) cut a mask equal to the image area;
3) position the mask on the photographic paper using the easel blades as the guide;
4) lift the blades while ensuring the mask stays in place;
5) expose the borders;
6) remove the mask;
7) lower the blades and then adjust them slightly to increase the image area;
8) expose the image, which should bleed slightly into the black borders.
It might help if you could put something tacky on the back of the photographic paper, to ensure it stays still. Some easels may be unsuitable for this, as they may have a tendency to move the paper when you move the blades away.
Actually cutting the mask may be finicky. A mat cutter might be helpful.