Well, the first thing you need to be aware of is that level 1 ADC Garen with Dark Harvest is pretty strong and you can get some unexpected level 1 kills on bot and most of the time people tend to loose their focus if a Garen ADC kills them head first in the match, so taking some unexpected quick actions as Garen ADC can result in an strong early lead.

Another important information: If you feel that you're week in the lane don't get into fights, try to farm as much as you can and if you inevitably get into a fight with no hope, always try to take the Dark Harvest, because if the enemy doesn't really care about wave management and, for example, instantly backs after kill, it might be more worth to you as you have the Dark Harvest stack

Whenever I feel like I'm weak in lane, I either play very safe until level six, or wait for a gank to get at least the aforementioned DH

In early-mid game I tend to leave my lane a lot after I have obtained the mighty power of Galeforce, and I usually help a lot to my teammates to have as many DH as soon as possible, and also to help establish a lead in all lanes. If you want to act like this than you need to restrain the usage of you R as most of the time it is not the best thing to go for the kill instead of the assist as sometimes, a solo laner needs the kill more than you do, so I advise you to only use R on other lanes if you are confident that one of you would die or if the laner cant confirm the kill

Now in the mid game phase there are the team fights. Now this is a pretty hard thing as a Garen ADC so it takes some getting used to and a good tactical overview of the situation. Most of the time you should act like some kind assassin, try to be invisible in team fights, I always try to go in a bit later after engagement as Garen ADC packs a lot of damage and if I'm unseen I don't get focused and get like 1-3 kills at least. So to conclude, I would say that you should be aware of your limits (that's why I added the sentence "it takes some getting used to") and you have to be careful with your dramatic entrances into the fights

When it comes to late game it is sometimes good for you, but if you play against hypercarry champs like Twitch or Jinx it is better to avoid late game, not because they will easily get the better of you but because if you kill them you will inevitably lose a lot of HP so you can't help your team in a fight shortly after this and you have to deal with the rest of the team somehow

To achieve the best late game, try to farm a lot to have a full build ASAP and get as many DHs and there's a great chance that you will become an indestructible monster for the late game phase

So to conclude my little and not so brief guide I would to thank you for even clicking at a Garen ADC guide and I wish you loads of fun with this super build!

An important side-note: this build is viable in jungle as well and if you plan on playing this in jungle I'd advise you to follow the Red-Raptors-Krugs-Wolves-Blue-Gromp route