What got me from 900-1100 was playing carefully and evaluating my move before I played it(can this piece be taken/is there a mating strategy for my opponent), to prevent blunders
What got me from 1100-1400 was learning two solid openings (Ruy lopez for both sides/ Grunfeld against 1.d4)
1400-1600 was doing hella tactic puzzles, and putting myself in my opponent's position and looking for his tactics
1600-1800 i have no idea, just practice i guess
John Bartholomew's strategy videos and Saint Louis Chess Club opening videos were probably the best resources for me to learn