Just write your story top to bottom. When you want the reader to decide, put the cursor where the text should split, open ⋯ and pick Fork here: what comes after becomes path A, and an empty path B opens for you. Random event splits the same way but the dice decide. Every path has a code (A, B, A-B, A-P for "passed", A-F for "failed"). Finish each path with The End or Go to another path. Click a path's code to open it; the others fold away.