Most of you will likely assume that you already know how to leash and this advice could only help new players but i can assure you in my experience regularly playing at a variety of levels bronze-diamond that it is very common from all those elos to see players having no idea how to leash or simply not caring about it.
A leash is a very important tactic to use in League of Legends if you want to maximize the effectiveness of your jungler. I’m sure everybody that has played this game has experienced either giving or receiving a leash as it happens in every single game(or at least it SHOULD). You should look at leashing as step 1 of playing each match, and if you’re not doing step 1 correctly then you’re putting your team at a disadvantage almost immediately.
First thing I want to mention is Relic Shield. If you are playing support and have relic shield then you should be tanking at least 2 autos from the buff, you’ll heal most of that back as soon as you get to lane and use your stacks and it saves your jungler from a lot of damage. Also, if you tank 2 autos then trade agro you are preventing more than 2 autos from hitting your jungler. If the trading is done perfectly you can tank 2 autos but actually prevent ~4 autos worth of damage by cancelling the animation from the buff. This is saving your jungler from an immense amount of damage at very little cost to yourself.
If you have hard CC you should be using it, as long as taking that ability level1 doesn’t hinder your laning phase. Things like morgana binding, leona’s stun, alistar knock up. These CC will either prevent the buff from hitting your jungler or at least make it much easier for them to kite the buff.
This one should be a little more obvious but I’m sure there exists a player that doesn’t know this. The best leash comes from janna/soraka type champions. If you do not heal or shield your jungler on the leash you are effectively griefing. Intentionally not using your champion’s kit to limit your teammate’s ability to play the game.
In my experience ADCs are much worse at leashing than supports are. Whether it be from not caring about the leash or not understanding what they are supposed to do is not for me to say but i don’t believe that it is uncommon for ADCs to be less willing to leash.
USE AN ABILITY. Yes you’re an ADC and you’re used to your autos doing big damage but all you have is a dorans blade right now so give your jungler an ability. Of course we can’t generalize and say ALL adcs should use an ability, Jhin, sivir, caitlyn for example can be set behind in the laning phase if they show up with less mana. Jhin has his 4th shot to give an enhanced leash and caitlyn has her headshots to help so they have an excuse for not wanting to leash with an ability but sivir pretty much just doesn’t have a good option for leashing.
Except for the fact that many of these champs could take manaflow band, also including varus, ezreal, xayah. IF you have manaflow band then there is no excuse for not giving an ability to your jungler while leashing.
The biggest exception to the rules of leashing is blitzcrank. It is much more effective for blitzcrank to walk to the enemy buff (assuming the enemy started botside) and pull the buff over the wall causing it to reset and delaying the jungler a large amount. Potentially even causing them to not get a leash since they will effectively have to kill the buff twice.