The Field Dog Jr/Junior hunter is basically an instinctive test for the dog. However, to prepare, your dog should have had exposure to birds and point them, should know the commands "Come" or "Here", should have been exposed to gunfire, and should have run with another dog. Field Test in Canada are run with one dog alone, Hunt tests in US are run in braces-2 dogs at a time).
If possible, you should also expose your dog to horses as the judges are sometimes in Canada and most of the time in US on horseback.
The most common errors novice handlers make are:
1) Many inexperienced handlers don't cover the bird field. Use the ENTIRE bird field, not one corner of it. If there wasn't a bird in the area you are in the first time, there likely will not be one the second, third or fourth. I've seen many, many handlers THINK they know where the birds are planted, but if you do not cover the field you may end up birdless.
2) Walk at a good pace that is comfortable for you. In US, if you are braced with someone who can't keep up or someone that goes too fast for you, the judges will split and hold you (or the other person up) if you get to the bird field too early.
3) Teach your dog to respond to a whistle. Start as a puppy, one whistle “Go” and when in the field means “Change direction”, two fast whistles “Come” Don't nag or hack your dog all the way around the course. There is no reason to be hollering at your dog the whole time. In the case of fieldt/hunt tests, less is better.