The Equine Canada rule concerning entering and exiting the hunter ring is the following:
7. Circling a horse once upon entering the ring and once after completing the course is permissible, but any other circling, except to retake a fence in the case of a runout or refusal shall be counted as a disobedience. If an audible signal is used, this rule applies from the time the signal is given.
This means that if you circle more than once before beginning your course, you will be eliminated. You may also only circle once upon finishing, so don't endlessly circle before doing your downward transition.
Most of the time, the course designer will create a course that finishes going towards the in-gate so that the ring will run efficiently. This usually means, given the usual hunter course set-up, that the first jump will also be headed towards the in-gate. This means that you have two options upon entering the ring: you can either enter the ring, trot diagonally across the ring, pick up your canter and then go directly from there to the first jump, or you can insert a circle at the far end of the ring before heading towards the first jump. Let's look at the various options:
This is a good plan. When your first jump is headed towards the in-gate, there is really no need to circle since you've had lots of time while crossing the ring to pick up your canter and get going. Most hunter riders would enter the ring at the trot and then pick up the canter when they hit the outside track. The line of entry can be adjusted based on whether or not you want your horse to have a close look at any of the jumps on the way in as well as how much room there is between the jumps and the rail. You can also enter the ring at a walk instead of a trot or adjust the location of your canter departure.
Here, one full circle is completed before the horse exits the ring. This allows a smooth finish to the round without breaking any rules. It also gives you the chance to show your horse off a bit - loosen the reins at the canter and give him a small pat. Make sure that you exit the ring at a walk; leaving the ring at the trot or canter is dangerous!
This course shows you what not to do. While it is legal, it will take away from your round rather than add to it. In this course, the rider trots in across the diagonal just like before, but this time she circles before heading to jump 1. Since the rider has already taken the time to get from the in-gate to the opposite end of the ring, doing a circle is unnecessary and will slow the day down for the judge and everyone else.
Here, after the last jump, the riders exits the ring immediately without circling at all. While this is, again, legal, it will give your round a very abrupt end and detract from your previous flow unless it is done for a handy round (and it needs to be done very smoothly to benefit you even then).
We have established that it is best not to circle before starting your round if the first jump is headed towards home, but what happens if the first jump is headed away from the in-gate?
Here is an ideal plan for such a course. You will enter the ring, immediately put yourself on a large circle and do one circle only before heading to your first jump. This circle will give you the opportunity to establish your canter without taking a tour of the entire ring.
This also shows a good finish to the course. The rider comes off the last jump and does a single circle before exiting the ring at a walk.
This is probably the worst course plan you could come up with if your first jump is headed away from the ingate (again, unless it's a handy round and done very smoothly). Because the rider has not circled, there has been no opportunity to establish the pace and the judge might even miss your first jump if he is looking down, expecting for you to circle! There is a time and a place for such entries, such as in equitation classes where you want to show off your ability to establish the pace immediately. There is no place for this sort of a start in regular hunter classes.
This course also shows the same abrupt finish that we saw earlier. Even if you have finished the last jump, you are still being judged and you should make the end of your round count.
Now that we know when to circle and how you should never do more than one circle at a time, how big should you make your circles? A general rule of thumb is to use most of the empty space at the end of the ring. If you make your circle too big, you risk nearly knocking over a standard or two. If you make your circle too small, your course will lack flow. Usually the course designer will arrange the jumps in such a way as to give you plenty of room for a nice big round circle.