Takes patience and practice, mate.
With modern sonar, you're usually detecting your target from a long ways off (sometimes out in a convergence zone, over 30 miles away), and it's impossible to estimate the target's range and course right away. You need to take plenty of time, and make lots of course and speed changes. As you do, the number of possible firing solutions will drop, until you will eventually have a pretty good idea where your target is and where he's heading. Once you're in closer, don't hesitate to further enhance your solution by using your two other passive sensors (the ESM, and your periscope). Marking your contact with these sensors will also reduce the number of possible solutions, and you can get a much better idea of a target's course.
Cheers.
