My very first try at making laundry detergent was acceptable but I didn't feel as wowed by the whole experience as I wanted and for a short period actually went back to using store bought (yet still locally made) laundry detergent. This is a fact that I feel worked out for me in the end because it left me with more suitable containers for my home made laundry detergent than I was previously using.
To begin you will need a bar of Fels-Naptha soap (which I shamefully purchase at my local WalMart), Borax (also purchased at WalMart, it can be found in the laundry aisle), and one of both of these optional ingredients I have also been using - tamanau oil and tee tree oil.
Begin by grating your bar of Fels-Naptha soap. I like to grate as finely as possibly so it will dissolve quickly and easily. Doing this over a large container like a bucket seems to be helpful because it catches shavings that might otherwise just sit next to you on your chair.
Once it's grated, if it isn't already in a large bucket or container, put it in one. The first times I made my detergent I used the largest pot I had available in the house.
With your grated soap now in your bucket add a bit of hot water. I run my tap and let the water get as hot as it can. I add just enough to swish it around really well and dissolve it. Stir vigorously with whatever you've got on hand - I usually use a slotted or perforated spoon to get everything moving around really well.
Your borax should now be dissolved into your mixture. Here is where the optional portion comes in. I add either 100 drops of tea tree oil or 50 drops of tamanau oil followed by 50 drops of tee tree oil. These oils have amazing anti-fungal and antiseptic qualities and I really feel they give my soap a phenomenal boost.
Finish filling your bucket with hot water - like I said I work with a three gallon bucket, if your container is smaller your soap will be a bit more potent or even thicker, we can take care of that later. Give your laundry soap another really good stir and then set it aside overnight.
When it's ready you should have a very thick gel in your bucket. When I try and finish up sooner than 24 hours I check to see if the mixture is ready by just jostling the bucket and seeing how solidly gelled it seems to be. If it's relatively immobile (meaning no sloshing) but shakes a little bit like jello (not exactly like jello but close enough) your soap can be considered ready.
In the past I used spoons to stir and break up my laundry detergent, this left me with what seems to be a common complaint/problem people have with making their own detergent. To really break it up and get it the best possible consistency (in my mind at least) I use an immersion blender. If you don't have an immersion blender I don't see a lot of reason why you can't try and have good success using your regular kitchen blender.
For anyone who was using a smaller container to mix their laundry detergent you can solve the issue of how much water you originally had by just evenly dispersing your soap across the amount of container you intend to use. Because I'm not there with you and have no idea what size container you are using as a general rule of thumb I won't recommend filling your container more than half-way with detergent. Try all of this out on only your first container so that you have the best chance of ending this experience positively.
Now that you've filled this "mystery container" half-way with laundry detergent, fill it the rest of the rest of the way with water. Shake the container and look to see how happy you are with the consistency. The key for me really seems to be evenly dispersing the amount of laundry soap across the amount of containers I plan to use.
I owe a lot of the success I now have in making my laundry detergent to the immersion blender my mother so kindly gifted me (she wasn't using it.) Thoroughly blending your detergent yields the highest rate of success in creating a laundry detergent that isn't lumpy.