In this video I'll explain how to use the path signals in OpenTTD with two examples.

To summarise:
Path signals work by checking if a train can reach it's destination (the next signal along). If the path to that signal is obstructed, the train will wait.
One way signals only allow a train to move in one direction. These are pretty important if you want trains to stick to one side of a railway.
Trains slow down if a 90° turn is shorter than the length of the train, so it's a good idea to have the turns in a junction be as long or longer in order to maximise flow. (This does not count for 45° turns, or two consecutive 45° turns that even out).

What I didn't mention:
Unless you know what you are doing, you shouldn't put signals inside a junction, only on the outside. If a train stops half way through a junction, it can cause a complete jam.

I'll update the description with the answers to any questions I get regarding t his video, please comment if you need help (if you have my discord, just DM me).

Cheers!