1 bar Soap
1 cup Borax
1 cup Baking Soda
1 cup Super Washing Soda
Grate the bar of soap super fine - I use my food processor because there is no way I'd do it by hand. Mix well with the remaining ingredients.
Use 2 tbsp per load of laundry. And yes, it works in cold water.
Don't expect suds.
Most recipes I see call for Fels Naptha or Zote soap, but those are not vegan so I just use whatever I happen to have so long as it is a full size bar. It hasn't let me down.
You could add a couple drops of essential oil if you wanted, but I never do. This is also a good way to use up all of those little pieces of soap that fall apart in the shower once your bar is almost gone.
I like to use white vinegar instead of fabric softener and I use hydrogen peroxide instead of bleach.