First you will need to locate trees that have beans. The drought has made that a bit tough this year.
A few hints and warnings on harvesting Mesquite Beans. Remember, Mesquite bushes/trees have thorns. Long scratchy thorns. We all knows that, but, some of us (me) forget to exercise caution. I came home looking like I was wresting with a bobcat. Secondly, rattlers and scorpions as well as hornets and Africanized bees also like Mesquite bushes. Please exercise caution. I learned the hard way that hornets are very protective of their trees. Thirdly, pick beans that are not green, and try to harvest them off the tree. If they hit the ground, they get infested with little spider looking bugs that will swarm your arms immediately upon picking them up. A pair of pliers works well for picking the clusters of beans.
After picking these beans, a good washing is in order. A few will contain some bird poop, and you will find ticks, and other creepy crawlies. This patch only yielded a few small insects. Yes, I know I have a few green ones in the sink. I separated them out.
You will notice that these beans all have barbs on the the ends of them. A pair of scissors was used to snip off the barbs, and the end that attached to the tree. I highly recommend doing this.
After these are rinsed off, put them in a pan and dry them out at 170 degrees for a few hours. You must not skip drying these. If you assume they are dry and grind them without roasting them, you will end up with a gooey mess that will be near impossible to clean up. You know the beans are well roasted when they easily snap.
After these are roasted, I suggest breaking the beans into small pieces. I used a my Oster brand blender to crush these up into a flour. There may be a better way, but I have not found it.
Do the grinding in small batches, do not try to stuff the whole blender full. After they are ground, run them through a fine screen mesh colander. You will get a decent amount of flour, but will also get a lot of debris.
A note here. The flour that comes out is sticky. It is very sweet and is almost like powdered honey. Do not get this flour near water, and be aware that this flour will stick to the bowl and in the blender. I have also found that the best way to store this is in a airtight bowl in the freezer, but it does ok at room temperature. I am still experimenting with the flour, but have found that 25% mesquite flour to 75% wheat flour makes a good bread. Being that this flour is so sweet, I will be experimenting with making muffins and pastries. There will be a follow up.