I plan my comic in concepts and professional rambling.
Concepts are usually written in explanations to events or characters. I find that, when I do this, I am able to clearing see who my characters are and what is happening. That way, I've created several possible scenes in advance.
Two part of writing concepts deals with using music. I love listening to music while I write for my stories because I am able to picture a music video explicitly linked to my story. With a little editing, that "video" becomes a possible scene (if not several scenes). It's like writing an opera, basically
Now the "professional rambling" is just what it is. I write out everything I can think of on the fly. If scenes come to be and they seem to connect, and piece them together. Sometimes, I write out scenes where I need to add more to them (in that case, it helps to write "ADD MORE HERE" in big bolded letters so I know I need to connect parts. I also make sure to go back and edit things for grammar and comprehension. If it doesn't make sense to me, it probably won't make sense to my readers.
I also love to have close friends read over my plots and writing, so that I can answer any questions they have or edit parts that need some work. It's also good to have some extra eyes looking at your work. It's noted that your method doesn't have to look professional (unless you're showing it to professional industries and employers). For me, I write a lot of my planning in first person so that I know myself what's happening.