Use your imagination and break a job down into it's smallest parts the first time. Once you've done the job, hyperaccurate task timekeeping will help you in the future. How long does it take to setup & cleanup, hang a fan, change an outlet, fish a wire? Once you have these "actual" tasks in your records you do estimates like an assembly...3 ceiling fans, 3 fish wires, 3 switches, 1 setup & cleanup. Keep your time estimates generous. It's better to make more profit than less.
Keep tracking your time & check your actual time against your estimated times. If it's taking longer to accomplish a task, you need to correct that task time, or you'll lose money every time you do that task. It's OK to lose money once, but it needs to be corrected quickly.