There is a help page. Well, "training" page to be precise. However, it is out of date now. It talks about using the mouse pointer as an impromptu gun site piper, but we don't have mouse pointers anymore without the clipboard up.
All I can say is that direct mode is just a gun. You aim it as you will. Good luck and may the Force be will you. That is honestly the only way you will hit anything that isn't incredibly close by.
Land mode lets you click on the clipboard map, and then the gun will turn to that point, even miles away. Then WASD will let you move the point of impact around. A and D will shift it side to side, and W and S will lengthen or shorten the range. If you are shooting beyond visual range you will need another player to spot for you. This works with the fleet cruiser 8 inch guns as well, even with the carrier fleet moving around. Your shells will continue to land in almost the same spot on the map.
Sea Mode is similar to Land Mode, but point of impact of your shells moves with your moving ship. If you are in a shore battery, then Land Mode and Sea Mode probably have no difference between them (you are not moving). If you fire out to 7 kilometers to the fleet's 10 o-clock with the cruiser turrets, then the next time you shoot will be 7k out, towards the 10 o-clock position. Again, WASD will let you move that point of impact around. Sea mode is still hard to hit anything with this, but with some practice you can probably get good. It really only works well if the two fleets are sailing in parallel to each other, trading shots like old battlewagons from the battle of Jutland.