Bending lines are not easy. Do you steady and do the line in 7? Do you ride it more direct in a 6? How much bend do you want to put in the line? There are so many options, and a lot of times, you have to see how you ride the first jump to make your decision on the number of strides (that is unless you are jumping over 1.20m, then you better have a good plan!). The best way to practice bending lines is by schooling frequently at home. To prepare for a competition, you need to make the course tougher than what you would see at the show. For example, having the course include a double bending line rather than just one bending line will allow yourself and your horse to become more comfortable when this question presents itself at a show.
Increasing the difficulty of the course doesn’t necessarily mean making the jumps higher, it can often mean making the course more technical. The main focus of creating a technical course is to build confidence for both you and your horse, increase accuracy, as well as building on the fundamentals; therefore, making the jumps higher will likely prevent these points from being the main focus of the activity. At home, it's not about the height, its about the accuracy.
If you are hesitant to start with a full course, it's best to begin with a S-shape pattern of poles on the ground with about 6 canter strides between them. You can start by trotting the poles and changing your bend between. After you are confident with trotting, then canter the poles in a bending S-shape and do a flying change over the middle pole. After practicing this exercise and feeling comfortable with the poles, you can begin to move to a course.
This video shows one of our favorite examples of a great course to establish more confidence in preparation for any bending lines we may be faced with at a show. Pay attention to jumps 4, 5, and 6. Between jumps 4 and 5, the rider did 6 strides and between 5 and 6, the rider did 5 strides. For this exercise, you can do either 7 and 6 strides, or you can go very direct (the rider put a nice bend between jumps 4 and 5) and do 5 strides and 5 strides. Feel free to replicate this at home!