We don't mix our feed ourselves, so I am not the best person to ask. One thing I would advise is to get yourself set up with a few feed bins or containers to keep all the individual ingrediants in.
If I were mixing my own mix, I would probably use soya bean meal to add protein, flaked maize and flaked peas, some oats or barley (not whole, probably crushed or flaked), linseed flake, locust beans. You wouldn't have to use as many as those, but some of those anyway. If you google goat food ingredients you will find different goat feeds and you can look at what their ingredients are. Most feeds are moistened with something- molasses or wheat oil, but I am not sure where you would source thise from, and I think it would make it more difficult to mix.
Beth