This advice is only really for players who are serious about upping their Overwatch game, because practice drills are boring. And there's a very easy to way to improve your aim(albeit slower) while having fun, and that's playing Overwatch.
So if you're fully committed to improving your aim and are prepared to go through boredom to do it, the best way is by using a $5 piece of software called AimHero. I am in no way sponsored by AimHero, I just really like the software. It has a wide variety of game modes that all focus on improving different parts of your aim, for example, flick shots, tracking and strafing.
If you don't want to use AimHero the next best way to improve your aim is by playing a headshots only gamemode in custom games, this method works a lot better than using Ana bots because you get used to landing headshots on actual players(never use the Ana bot method, it's a waste of time). It's also a lot more fun than using AimHero.
If you're looking for more information check out overwatchaim.com. They have a bunch of useful information about the subject that will really improve your aim.