I wouldnt concentrate so much on the pattern as I would the fundamentals of what it takes to barrel race.
While it is important for you horse to eventually have the pattern memorized, your horse should be 100% broke to ride in every way BEFORE you even start patterning on the barrels.
-The horse should neck rein.
-The horse should ALWAYS stop on cue.
-The horse should back up.
-The horse should know both simple leads and flying lead changes.
-The horse should be reponsive to leg cues (move away from pressure).
-The horse should be able to move in a relaxed manner at the walk, trot, and gallop.
-Basically, the rider should be able to move the horse's body in any way they please.
So make sure you have general training taken care of before you hit the pattern.
There are lots of great books and DVDs out there on barrel training such as Charmayne James, Marlene McRae, Sharon Camarillo, Sherry Cervi, Martha Josey, and many more.
But main idea is to keep your pattern a bit larger (bigger circles around the barrels, and more exagerated) because a new horse to the pattern is going to need more room when they are starting out. Later in training as they are becoming seasoned, the pattern will "suck in" because you are going faster.
And ALWAYS take your time and take things slow. The easiest way to ruin a pattern is go faster than you are ready for. If you try the pattern a little faster and your run falls apart, you need to slow it back down.
Also, the best way to sour a new horse is to over-do the pattern. There are tons of things you can do to practice barrel racing fundamentals without actually doing the pattern with 3 barrels. Roll backs, figure 8's, perfect circles, spirals, and things like that.