It's not always easy to tell from looking at a ship whether it is perfectly side-on, or whether it is moving slightly towwards or away from you. Alternatively, for ships that are manoeuvring rapidly, fire your turrets in close succession at slightly different locations, to give you a higher chance that one of them will hit. If you're not already firing one turret first to see where it lands and then adjusting, try to do this.

For anything other than that, though, it really does require some experience and practice, as well as knowledge of ships' speeds (and estimation of whether they are going at full speed or less) So, if a ship is moving at that speed perpendicular to your view, then you can read off the shell travel time and aim at that number. If you have the default static crosshair and are fully zoomed in the numbers correspond to shell travel time for a ship moving perpendicular to your view at 20 knots. If you have the dynamic crosshair enabled, the numbers on it correspond to the shell travel time for a ship moving perpendicular to your view at a speed of 30 knots. Make sure you have 'alternate battle interface' enabled, as this gives you a readout of the shell travel time for your aim point.

How to aim static in world of warships