Ask HN: Learning Proper Planning

Most of my career I worked on teams in which we roughly sketched things out or drew on a board and often found our way by coding ourselves to a solution. Now I work alone, remotely as a freelancer and after a recent bout of RSI I want to be more mindful about how much time I spend coding vs properly exploring a solution. I understand not everything about a problem can be felt out without implementing some of it but I'm sure I could be more effeicient. So what tools and techniques should I look into for properly planning a software project?