I have a recipe that I use and it generally turns out quite well.
I use about 1 oz of grated bar soap (usually my homemade lye soap, but I think any real soap - not detergent bar - would work) and dissolve it in about 1 1/2 cups of boiling water. You have to make sure it's completely dissolved, or you will get chunks in your soap. I let it sit and cool overnight - if it's too thick, I heat it and add more water. Then as it's cooling again, if I had to reheat it, I add a few tablespoons of vinegar or lemon juice to help the soap cut grease, and sometimes a few drops of essential oil for smelling-goodness. Just put it in an old dish soap bottle, or even a fancy glass cruet - and be sure to label it!

Don't want a vinegar and soap salad dressing or something like that!
I hope that this works for you!
Amanda