Play on lichess.org. Play correspondence with a long timeout and take your time -- make sure you are making the best move you can think of, every single time.

Play on lichess.org. Play correspondence with a long timeout and take your time -- make sure you are making the best move you can think of, every single time. Use their engine analysis feature after each game to see moves that you may have missed. Also I got a lot out of Jeremy Silman's books, in terms of developing strategy. (disclaimer I'm not actually very good)