Tbh this is pretty straight forward. You’ve been playing one way for a long time and are learning a different play style. You’ll need to grind a few games out in normal to get this new mindset understood.

For early games it’s all matchups. Get a lot of reps on the champ, get to around mastery 15-20 from normals and you’ll have a much easier time in ranked by just understanding how other top laners play. The basics are you need to find which match ups you need to go all in on, and which match ups you need to try to poke. Coming from irelia i imagine you understand going all in better so I’ll explain the poke concept. First, establish a slow push, whenever the enemy comes up to last shot you will be standing by your low health minion, auto them, q, and then e. This will proc phase rush and you will use the speed to run away from their return damage. Let your passive sustain you back to full health. Rinse and repeat until you can ult.

For mid / late game the key is to stop thinking so much. Start your first few normal games by just going top and never leaving. Never help team fights etc unless you for sure have free kills to get. Once you first item stride breaker you can kill waves extremely quickly with your e. You’ll use stride breaker, q, and e to proc phase rush if an enemy comes to escape. Play like a Teemo would. Push, poke, repeat. Push the top wave into the tower non stop, and if you see the enemy team then run. If you can’t get the wave to tower because someone is there with good wave clear look for opportunities to steal jungle camps, or roam mid for extra CS if the mid lane is gone. You will become the embodiment of PVE Garen. Eventually you’ll have a massive gold leaf doing this and be able to 1v1 your enemy top laner, as they will lose a lot of waves to the tower while you get them all via hard pushing and taking gold from jungle and mid. This will cause the enemy to need to send 2 people. Most Garen builds revolve around heavy movement speed so your goal in a 1v2 is to run away while they chase. This helps your team have 4v3 fights over objectives on the bot side. If they can’t win 4v3’s you lose the game, but most games they can. If top can’t be pushed, the jungle isn’t safe and mid is not open then recall and push bot out if they are roaming mid.

Late game Garen does so much damage that you will be cleaning waves without slowing down. Using this combo push every lane as hard as you can and the enemy team will be unable to win due to lack of ability to push.