foaming soap dispenser
1. Add water to the soap dispenser, about 3/4 full. Keep in mind the size of your container and the size of the pump when the top is on.
2. Add several pumps of soap or 1-2 squeezes of a large refill container to the water, enough so that once shaken (see step 3) it's the same watery consistency as it would be if you bought the foaming hand soap as is.
3. With one hand covering the top of the bottle, shake it so the water and soap mix. You don't want to put the top back on before shaking because if water gets in the pump, it won't make foam anymore.
4. Screw the top back on and test your soap. It may take a couple pumps to get the right foaminess. (Did you know foaminess is a word? I looked it up to make sure it didn't need quotation marks.) If it's still not quite right, adjust soap and water as needed (this part may take some experimenting).
When I first starting using this method, I stuck to clear liquid hand soaps. After experimentation I've found it also works with the moisturizing type with a pearly sheen appearance (as pictured), but these soaps do make it a bit harder to get the right soap:water ratio since they're thicker--so be careful not to add too much.
Source: Myself, basically. Years ago my mom bought some soap pumps from Pampered Chef that caused liquid soap to foam. All you had to do was add soap to a certain line, add water to the top line, and shake. (Yes, my instructions have you put the water in first...I think it sort of "pre-mixes" the solution for you.) So I figured I'd try it on an empty Bath and Body Works dispenser one day, and it worked!