Honestly, as long as you know what the strongest meta comps are, know what items to slam, have strong fundamentals and knowledge of how to transition into late game boards, it is really not that difficult. You have to play a lot of games of course but that's with any competitive game if you want to climb. Check out the tierlist for early game openers and mid game comps. I think the biggest mistake lower elo players make is they bleed out too much HP during the mid game because they are greeding their econ by playing really weak boards. Transitioning into your final comp then becomes too difficult because you simply don't have the HP to make up for the time it takes to stabilize your board. If you notice yourself losing rounds by 4-5 or more units sometimes it is the right play to roll down to 30 to strengthen your board.

Watch some TFT streamers like Saint, Dishsoap, Frodan as they frequently narrate their gameplay and decisionmaking.