When I was stuck in the 1200-1500 rating range I made this huge mistake that was holding me back, when I look back on it now I understand why, but I've been noticing that other 1200-1500s have been struggling with it too and I know the pain of being stuck at that rating range. The tip is STOP BEING LAZY. The main way that I was lazy was not calculating correctly, I would see a "good move" (something barely playable that I found in 5 seconds) and play it without a second thought, the only thing keeping me alive was my intuition. Some common examples that I've seen are people trading an excellent piece such as a knight outpost for a bad bishop that is blocked by pawns, people do this because they are too lazy and don't want to calculate for themself and their opponent's responses, there are other ways you can be lazy: such as not wanting to study or start studying (this will hold you back at every level), and even with things like game review (how do you expect to improve if you aren't directly addressing your problems). This is the most important tip to improve, and I hope it helps you.