This will work using any brand, but I like Method foaming hand soap.
After I finally used up all the soap in the bottle, I poured about an inch of the liquid soap into the empty bottle. (On one blog, they said to use 1 tbsp, but when I tried that, it was no where near enough soap. I don't like soap that is too watery & runny).
Next, add water, but don't fill it all the way to the top - when you put the pump back in, it will spill if you do. Some people say not to shake it very hard, but I wanted to make sure the soap and water mixed well, so I shook away. It will cause bubbles at the top, but the bubbles will go away.
You're all set now - wash away!
Since I just tried this, I'm not sure how long the bottle of gel will last, but it's looking like I'll get about the same number of bottle fulls as the refill of the foaming soap. So, that's a savings of almost $2 per purchase. And depending on the brand (I think some brands have gel refills that come in a bigger bottle), you may get even more of a savings. This will certainly add up over the course of a year!