When I first started writing, I really didn't plan. All I would do is get the basic idea and go for it, having no idea about characters, setting, plot, or anything of that nature. But now things have changed, and I plan with more attention.
I start with the basic idea: characters and the situation. I give the characters their goal, or where I want to story to end up. Then I write a very loose outline of the chapters, where I figure out what happens in each chapter, so I know if I have filler or if every event adds to the plot/develops the characters. After that, I give a simple order of events to happen in each chapter before I write it (so only one chapter is planned at a time), but any further details are completely unknown, and I'm still never sure if there will be a change to the plan until it's written.
Right now, I can manage a chapter each week, which includes planning the chapter and writing it. (Editing comes later.) But the chapters for this fic are on the short side (eleven pages). Other fics have much longer chapters, which I know will take longer.
I should mention that I have one story that I keep completely unplanned. When I write that one, I don't know what's going to happen. All I know are the characters and where they're going to end up.