Easy way to sharpen disposable razors to save money.
Supplies:

Disposable razor (in this case I used a 2 blade, but 1, 3, 4 or 5 blade razors can be used). The blades of the better razors like Bic/Gillette seem to work better with this hack. The blades of cheaper razors do not seem to sharpen well with this method.
Canvas tool apron (used an inexpensive one from Home Depot, SKU #324655)
You can also use a piece from a canvas drop cloth (be sure to use the canvas side and not the plastic-coated side. The canvas drop cloth seems to work a little better than the canvas apron.

Step 1: Move Razor With Blade

Fold the canvas apron along the vertical ("Y" axis) until it is about 2 inches/5 cm wide. I folded it in half, then in quarters, then 1/8.
Place razor on canvas, and move razor in the direction WITH the blade.
Repeat about 10 to 15 times to sharpen the blade.
Repeat process about once a day to keep it sharp.

Step 2: Note on Razors With Extra Plastic on Sides of Blades

Some brands of razors have extra plastic on the sides of the blades (circled in picture). This extra plastic causes the blade to be elevated slightly and may not make solid contact with the canvas to sharpen properly.
Run the razor in the direction of the blades, but keep one side of the blade slightly over the edge of the canvas. This will allow the canvas to scrape against the blade to sharpen it. The arrow in the picture points to where the blade is hanging over the edge slightly.
In order to get the other side of the blade, rotate the razor 180 degrees and then make sure the other side of the razor is slightly over the edge. Then sharpen the razor again to make sure you get the side that was not sharpened the 1st time.