Brain tanned leather is, at the same time, both one of the most labor-intensive yet easily done skills. It requires no huge monetary layout or knowledge of secrets. Just a ton of elbow grease and the know-how.
There are individual preferences, as well as techniques, but the basics really don't change;
Scrape hide clean. Hair on or off as per need.
Work brain slurry into hide. Make it penetrate.
Stretch, pull, cuss, pull some more, stretch. Scratch head, take a bathroom break. Resume stretching, pulling, cussing over and over until the hide is dry. It is critical to work until its dry. If not, when it does dry it will be hard rawhide, unsuitable for clothing applications. The hide is now tanned.......but you're not done.
If the hide gets wet it will become stiff rawhide when dry again.
Next step;
Smoking the hide. Smoking does not tan the hide. It renders the hide unable to become rawhide (but it doesn't break the fibers down- that's what the stretching does). If a smoked piece of brain tan becomes wet and dries, shaking it out or wearing it (if an article of clothing) will soon return it to its velvety state.
So, the process is twofold.
My BEST advice I can give is to have a couple hides already scraped and soaking in brain slurry. Invite a half dozen friends over and have them pull all day. Beer and pizza help motivate.
And your hands will get super soft.