I've been refilling liquid soap bottles for a few years now, with a method I'll describe at the end of this post. Recently, however, I heard an "eco-minute" tip on Seattle's AM 1090 which recommended taking the slivers of bath soap, adding them to an empty liquid soap bottle along with a few marbles, and filling the bottle with hot water. Shake the bottle well, and let it sit for a few days, and voila! Liquid soap.
Always looking for new ideas, I tried it. The soap created was gelatinous and difficult to pump out of the bottle, and after about a month, the pumps on all three liquid soap bottles in my house (in 1.5 baths and a kitchen) stopped working.
OK, so back to my old method! First, I use my bath soap slivers until they're gone, by rubbing them into a washcloth. So I don't really need to use them to create liquid soap; they won't go to waste in my house.
Second, here is what I do to make liquid soap. I buy foaming liquid soap dispensers, which reduce the amount of soap you use by mixing soap, water and air to create foaming bubbles. (I've just purchased 3 new bottles to replace the now useless ones! I bought Kiss My Face peace soap, made from Castile soap; an 8-oz bottle costs $2.99 at Grocery Outlet)
When the bottles are empty, I refill them with the following mixture. I buy a 24-oz. bottle of Earth Friendly Dishmate natural dish liquid at Grocery Outlet for $2.49. I use dish soap instead of liquid soap because it's very difficult to find natural liquid soap in bulk quantities. (The exception is Dr. Bonner's Castile Soap, which you can buy by the gallon. However, a gallon costs about $45).
I empty my 24-oz dish liquid into an empty gallon-sized SoftSoap refill bottle, and then fill it with warm water. Here's the secret of foaming liquid soap: the ratio of soap to water is 1:4, which makes it much more cost-effective than using a regular liquid soap dispenser. So by filling a gallon (128 oz) refill bottle with water, I achieve close to that 1:4 ratio (24:96, or 120 ounces total).
I gently shake the bottle to mix the dish soap and water and let it sit. I have found that pre-mixing the soap and letting it sit for a while produces better suds than mixing the soap and water in one of the smaller dispensers just before use. When my small dispensers are empty, I give the big bottle a gentle shake, and then refill them. I can refill three liquid soap bottles for about a year with this mixture for only $2.49, along with the initial purchase of the foaming pump dispensers.