We prefer to keep it as a powder. Making a liquid takes more effort, more space, and it likes to separate. We probably use a little more soap in the end but it is more convenient.
Washing soda is basically dehydrated baking soda. This can be really useful to know if you cant find it. I don't think the price difference is that great, but just in case. To make it yourself just bake a thin layer of Baking Soda at 400 degrees for about 30 minutes. Look for the texture to change from shiny crystals to a flat (as in not shiny) powder.
You can in fact use this powder in an High Efficiency (HE) washing machine. This soap is not overly sudsy, which is the main issue with HE washers. In fact the amount I tell you to use below is based on my experience using an HE washer, you may need to add more soap for a traditional washing machine or if your water is extremely hard.
Here it is. 1 Cup of Washing Soda, 1Cup of Borax, and one Bar of Fels-Naptha. I heat up the Fels-Naptha in the microwave for about 30 seconds to soften it a little (which make the house smell awesome), then grate it using a cheese grater. Add all that to a food processor and viola, Laundry Detergent.
You can add all kinds of other stuff if you want, but I think they are all gimmicky to be honest. Oils are the least gimmicky. Five or so drops of essential lavender oil is supposed to make it smell like Tide. My wife loves the smell of Tide, so if you find a perfect recipe let me know.
Just put 1-2 Tablespoons of detergent in the front (with the clothes). Balance the amount of detergent to the load size.