Hi, I'm new to the Forum, but do have a bit of dressage knowledge. Though I have to admit that the tests can be extremely difficult to interpret! Now picture a retangular arena where the entrance gate is at A, going straight in, the middle is X and straight ahead is C. Now pretend you're standing at X, directly to your right (on the rail) is B and to the left (on the rail) is E. Now pretend you're standing just inside the gate at A, directly to your right (on the rail) is F and the left is K. And lastly, go down to C and directly to your right is M and left is H. There are other letters in between, but they're used for the more advanced levels.
Now the F X H change rein would mean (if I'm picturing this right!) you must be coming past the A entrance gate from a counter-clockwise direction to that corner letter F where you turn and start heading across the ring diagonally through the center X (where you would change your posting diagonal if you are trotting) and on to the letter H where you would then be turning and tracking clockwise.
Does that help and clear things up?
In the basic level tests just work on travelling straight (shoulders and haunches in a line), keeping your pace consistent and as you said your circles round. You're usually given a few strides for transitions, so you don't need to be able to pick the canter up immediately at the required letter. It can take a couple steps.
Well I guess that's all. I hope I didn't confuse you more!