1, tracking. Learn how to track your opponents movements and predict them in order to make the best out of long range, safe sniping.
2, flicks. This is much more difficult. Essentially, you'll just be molding your sensitivity and movement until you get it down so well that you can move your crosshairs quickly past someones head/body and pull the trigger at the perfect moment. When you're first learning it, it's best to do it on training bots in the practice range just to get a good feel for it, then (at least for me) move to a custom game, you against a team of 6 hard ana bots with headshots only turned on in skirmish mode in order to practice it with one of the smallest head hitboxes in the game. (a good place to practice you tracking as well)
3, and most importantly, positioning and movement. Never stand still, never stay scoped in for an excessive amount of time, always be looking for new places to hide in a match to catch the enemy off guard, place your venom mine in tactical spots in order to either inform you of an enemy trying to flank you, or to give you the drop on enemies coming straight ahead. Practice grappling into the air to get a pick before you're actually set up (one of the most difficult things to do as widow)
Pretty much the basics right there