LoL is 90% knowledge and 10% technical skill to be good. You can not improve if you do not know the champions: their skills, staves, cooldowns, damage, ratios, duration, controls, runes, objects, masteries, movement speeds, speed casting, attack speed, weave and initial magic resistance, damage, match-ups, optimized setups. You can not learn everything at once, you will do it as you go, but you can not cut it. Playing all the champions gives a very big LoL Boost.
Knowing how to build your champion is one of the most important skills in LoL. Champions are nothing without objects because your objects determine your abilities. Having unsuitable items means having unnecessary skills for the team (typically a tank that builds damage). Building your champion in LoL follows more or less the principle of Stone / Leaf / Scissors (as in 99% of games). The situation in the game, determines which objects you must build to counter the objects / champions of the opponent.
Learn the meta of objects, situational objects, adapt your build to each part, swap the order of tier 1 objects according to his needs to achieve his core build effectively but different to each party, finish his objects when needed, optimize your purchases compared to opposite purchases are a set of skills that you must acquire as quickly as possible to progress. The best way is to play and follow the pros choices is irrelevant because they do not have the same needs as you in their game. They may play with HP Regen on a Bruiser because it plays tight, safe, while in silver / gold it goes in the air and it should take Attack Damage instead.
You have to trust yourself, do not be afraid to miss. You are not a Bronze, you are Syndra, one of the most powerful midwives. You are not a champion who has just been a counter-pick, you are the outsider of your team. You are not the least ranked of your team, you are the one who will show the other 9 players that Silver can be severe. You are not in 0/2 top because you know but because you have brought the enemy jungler all the time, which allowed your mates to break everything bot and mid. Trust, do not be stressed. Stay relaxed but firm. Be precise because your decisions are clear and you do not hesitate. You recognize your mistakes and that is how you learn, but you do not do much.
Know how to use Elo Boosting at the right time. Do not use Dash only to inflict damage, do not flash to kill while a mate is going to take the kill, do not ignite an already dead enemy, do not flash / barrier while you have no chance to get out alive Gank, not AoE ultimate for a single target in late game. There are obviously times when what I just said is not true, but you have understood the idea, you have to optimize your spells and not launch them "because they are available.
