How to learn how to cs

Take consume and before you attack a minion look at its health and your ad. It hit once it's on the same amount of ad you have or below it. It helps me most of the time

Go into an empty custom game and just watch the minion flow and how the lanes push against each other. Once you have a feel for this you can understand which minions to last hit in what order. Try this until you can get at very least 100-130 cs in 20 minutes. It isn't great cs for an uncontested lane,but it will put you in line with most people in the silver-gold range.

Also its a very basic tip , but since im unsure of how long you've been playing i'll give it to you anyway. When minions are under the tower you want to last hit melee creeps after they've taken 2 tower shots, and ranged after they've taken one. (some champions really struggle at this, especially early)

I agree with practicing csing in custom games but what you should really focus on is the first 5 minutes because that's when most people have the most difficulty csing (since you don't have many items to make csing easier.) Your goal should be 35 cs by the 5 minute mark.