Chop the bar soap into small pieces and place in a sauce pan. Heat over low until completely melted, then remove from heat.

Add the remaining ingredients and whisk to combine.

Pour the soap into the tube and let cool.

To use, apply stain stick to fresh stains and let sit for at least a few hours (or up to a week). Wash and dry clothes as you normally would.

Stain stick should last indefinitely, just make sure the cap is screwed on tightly to keep it from drying out.