Take some acting classes. One of your main duties will be directing actors. Get to understand their work and needs by stepping into an actor's shoes.
Write a script or find a script you want to direct. Go through the script numerous times until you start to envision every moment in your head. Break down the script into sections and make or write out character sheets. Track the progression of each character emotionally and physically. Think about how you want to express each of those elements. Share these character sheets with your actors so they understand what you expect from them.
Draw out storyboards for each scene and shot. Use storyboard software programs which will make the process far less complicated. Make sure you draw out each set, shot color scheme and special effect you can imagine. You want your entire crew on board with you.
Do a table read with the main actors to start to get a feel for the script being read aloud. Give beginning direction to the actors. This is your chance to explain the story as you see it. You can get feedback from the actors about how they see it as well. This collaboration is vital to the success of shooting days.
Do some rehearsals with the actors to get body language and movement addressed. As soon as the sets are ready, do on set rehearsals. This is called "blocking" where you give the actors specific movements and marks to hit at specific times. Rehearse the actors and call out their mistakes. Be respectful, but be firm about getting your vision realized.
Arrive to set early on set days to make sure everything is as it should be.
Look over the schedule for the day. This tells you which scenes will be shot that day and in what order. Read over the scene and any notes you have for it. Let the crew set up the background and lights. Bring in the actors. Walk though the scene once to refresh everybody's memory. Give direction to any actor who needs it or should rethink character choices.
Observe and take notes on every scene. Give the notes to the actors. Even if it is too late and the final scene was shot, giving notes to the actors may help to enrich their work in general...and for the rest of this project.