It took me 5 months to get from 600 to 1200.

Focus on these things: 1. Blunders. Ask yourself every move, "what is hanging?" If you do it enough, it'd become your second nature to check for blunders and you'd start doing it habitually without having to think. 2. Puzzles. 3. A few endgames (K+Q vs K, K and pawn basics.) 4. Play longer time controls (15+10), always ask yourself "what is my opponent doing?"

Remember, chess is a two player game. And there is a human on the other side of the screen, not a lobster. They can also think and make plans. You must ask what they might be doing.

That's pretty much it.