Go shopping for:
Borax powder (usually near the laundry detergent; green "20 mule team" box is common)
Washing soda (usually near the laundry detergent too; "arm and hammer" brand is common)
Bar of soap (I use Kirk's Castille; my cousin uses Dove Sensitive Skin)
Essential oils to scent the detergent if you wish (I use lavender and orange together; yum)
Find a 2-gallon or larger container to hold your completed soap (I use a 5-gallon bucket and double the recipe).
Got five minutes? Then it's time to make it.
Put two pots on your stove. Put four cups of water in one pot, six cups of water in the other. Start them heating.
While they're heating, grate your bar of soap (using whatever you use to grate cheese). When it's grated, dump it into the six-cup pot and stir periodically until dissolved.
While that's dissolving, dump 1/2 cup each of washing soda and borax into the four cup pot and stir until it's dissolved, too. When both pots are dissolved, dump them into your big container and stir, then add one gallon plus six cups of tap water, the essential oils if desired, and stir again. All done!
Put the lid on the soap and leave it for 24 hours. It should set up into a funky gelatinous mass. (Don't be afraid of it!) Use 1/2 cup per load in a top loading machine; possibly half that in a frontloader. That's it!