Make sure you're taking the shot when the scope is at 100% but if you're shooting too early then there is no way you'll get hs. However 144hz monitor should help. Also practice with ana with hs only mode.
Throw your Xbox/pc/ps4 in the trash, then light your trash on fire, in said fire burn an effigy to the gods of fps, using your own blood, write out your contract with the devil, ask for fleek ass sniping skills, then never play overwatch again you useless piece of subhuman garbage
After a while you can consistently guess where players are going to be at certain times of the match. Get some elevation, and if possible get behind them. Keep calm, line your shots and enjoy. As you get better you want to start flicking your shots.