This is an old post but I feel I should add my experience to help out other people. First off, the microwave method works absolutely great. My clothes came out ultra clean. I tried the, on the stove method first but that took two long. After trying the stove method I did what was suggested in this post and added some water to the baking soda (a small amount of baking soda that you would use for a large load); I am guessing around three tablespoons and put it in the microwave for 2 minutes. You want to add just enough water to make the baking soda just over pasty but not overly water. At the end of the 2 minutes you want the baking soda to come out dry and slightly hardened. All the water will have evaporated, and you can break up the now washing soda for your wash.
Now I have terrible eczema so what works great for me with this type of washing recipe is some baking soda, some of the microwave washing soda made from the baking soda mentioned above, and a tablespoon of salt. This laundry soap recipe works well. Cleans better than some name brands and is better on my skin. If you are allergic to Tide like I am, use this instead.