This idea certainly isn't my own, but I love it! It's so simple to do and saves a ton of money, plus it reduces waste and is more natural than mass produced detergents.
To do this you need:
Borax (approx $3.00)
Washing Soda (approx $3.00)
A bar of soap (I like Dr Bronners, which is more expensive at about $3.50. Fels Naptha is only $0.98 and also works great)
A 5 gallon bucket (or other large container)
4 gallons of water