All ya need is Borax (about $3), Washing soda(about $3), and a bar of soap. I use Ivory which I can get 3 bars for $1. Since you just need 1 cup of soda and 1/2 cup of borax, each 5 gallon batch really only costs a few cents.
First you grate up the bar of soap and cook on low heat in about 4 cups of water. You don't really want it to boil. I have discovered that if you let it continue to cook for a while after the soap is all dissolved it seems to help it thicken. I also add my cup of soda to the pan right when I remove it from the heat and stir it until it is dissolved. I have made some that never got thick but this step seems to help.
We have a 5 gallon bucket that we fill 1/2 way with hot water. Pour the pan on soap and soda into the bucket and then add the 1/2 cup of borax.
Fill the bucket the rest of the way with hot water and stir good. Let the bucket sit over night and it should thicken to look like this..
I have made some that did not get thick but it is still just as usable. It is the ingredients that clean the clothes and not the consistency. I have noticed that I use a lot more per load when it is thin. You can find tons of people that make their own laundry soap and they all seem to recommend a different amount to use per load. I say it really doesn't matter. I use about a cup per load. Some people (like Mrs.Duggar) also fill an old laundry soap container 1/2 full of soap and 1/2 full of water and then use that container when doing laundry. This would make your 5 gallon batch actually 10 gallons. I have done that but I usually just use it straight out of my bucket without diluting it farther and have not noticed any difference. Diluting it farther and using less per load would make this even more frugal but I am happy with $10 or less per year for soap. I just really can not see spending the money on store bought laundry soap again even if we get to a place in our lives that being frugal is not as necessary.