You must not begin your course until the judge has rung the bell (or whistle, or buzzer, etc.). This can occur at any time after you have entered the ring; sometimes it is immediate and at other times it can take several minutes. Generally, on schooling days that are not expected to run long, the judge will allow the competitors a minute or two to introduce their horses to the ring before ringing the bell. If the horse before you has had several rails down, you can expect to have some time to ride around the ring while the jump crew goes to work. If the previous rider has gone clear, expect to hear the bell almost immediately.
Once you have heard the bell rung a single time, you have 45 seconds to reach the start timers. The start timers will be accompanied by flagged markers that read 'Start' or 'Finish' (in some cases they might just read 'S' or 'F'). Make sure that the white flag is on your left and the red flag is on your right.
45 seconds is actually a lot of time when you think about it; your entire round, going around the ring several times, is generally between 60 and 80 seconds. Don't rush yourself to get between those start timers, but do make sure that you don't go over your 45 seconds because if you do, the timing of your round will start before you reach the timers!
As you walk your course, make a plan for what you will do with your 45+ seconds. There is time to show your horse at least one spooky jump before you get going, and planning a specific path will allow you to maximize what you can show your horse in that time period.
If you ever hear the bell rung multiple times or for an extended time, it means that you should stop what you're doing. It is the signal that you have been eliminated, but it can also be used when you're starting your course to ask you to stop and wait. Sometimes, for example, the jump crew forgets to reset a jump and the judge will ring the bell without noticing it and then have to stop you from starting on course. In such a case, wait for the bell to be re-rung a single time before restarting.
Here are the official Equine Canada rules that apply to the beginning of a jumper course:
FEI ARTICLE 203 BELL
1. The bell is used to communicate with the competitors. One of the members of the Ground Jury is in charge of the bell and responsible for its use. The bell is used:
1.1. to give permission to the competitors to enter the arena when the course is ready for their inspection (202.1) and to signal that the inspection time is over;
1.2. to give the signal to start; after the bell has been rung, the competitor must cross the starting line in the correct direction within 45 seconds; the automatic timing equipment must show a count down from 45 seconds on the scoreboard or other display beside the arena, clearly visible for the competitor. If the competitor has not crossed the starting line in the correct direction when the 45 seconds expire, the time of the round will start at that moment. Disobediences, falls etc. occurring between the signal to start and the moment the competitor crosses the starting line in the correct direction, are not penalized. After the bell has rung, crossing the starting line in the correct direction for a second time before jumping the first obstacle is counted as a disobedience.
However, under special circumstances, the Ground Jury has the right not to activate the start or to cancel the starting procedure, give a new signal to start and restart the count-down.
1.3. to stop a competitor for any reason or following an unforeseen incident and to signal to him to continue his round after an interruption (233);
1.4. to indicate to him that an obstacle knocked down following a disobedience has been replaced (233);
1.5. to indicate by prolonged and repeated ringing that the competitor has been eliminated.
2. If the competitor does not obey the signal to stop, he may be eliminated at the discretion of the Ground Jury (240.4.5) except where specifically provided for under article 233.2).
3. If, after an interruption, the competitor restarts and jumps or attempts to jump without waiting for the bell to ring, he will be eliminated (240.3.14).
FEI ARTICLE 204 COURSE AND MEASURING
9. The starting and finishing lines may not be more than 15 m or less than 6 m from the first and last obstacle. These two lines must each be marked with an entirely red flag on the right and an entirely white flag on the left. The start line and finish line must also be marked with markers with the letters S (= Start) and F (= Finish).