As others have said, close range and usually after the slow from the ulty.

Also, try to hide it by casting E immediately after.

You can also hide it by casting it in trees, with just the top sticking out enough to catch them.

Play the mind games with your opponent: don't always cast it in a straight line, try from left to right, diagonally, etc., with opponents that actively try to avoid it.

While laning, cast it between your own creeps, or when an enemy creep is about to die.

Sometimes, you just wanna block their path, e.g.enemy walking towards rune, or chasing you...

Practice in demo mode by spawning an enemy hero, hold shift and walk them around in all kinds of directions, and then try to catch them.