The biggest thing I have found while watching other people and working with horses myself is do it slow! Take a lot of time to get the basics then work your way up. I did it the other way around with my horse and spent 7 years paying the consequences. He will never be a great barrel horse because I made him hot and trained him poorly when training barrels. He has habits that are hard to break and make for an overall okay gymkhana horse once I fixed the major problems.
Before thinking of starting to patterns make sure the horse is well broke. So you have control over hind, shoulder, barrel everything. You should be able to bend and counter bend, stop, do roll backs, lead changes and the works...all before! It will make your life a lot easier.
From there its hard to say, I would suggest looking up books written by barrel racers. I like the Charmayne James barrel racing book, there is so much to know about rating, pockets the lead changes, patterns that's its hard to put down here. I will leave the rest to other more experienced people. The main thing I wanted to leave you with, is get the basics right the first time and take it slow!