Buy your early game item and core first, then go for situational if you need one. After you have finished your core and you have any situational you need, get one of the items to build from your Yasha. After that go for one unique attack modifier item, then sell your Mask of Madness if you have one. If you have done all of this you can start building luxuries.
Shrapnel is good to give vision, slow, and deal damage. Don't waste charges so you can use all of them during a fight.
A value point in headshot is good for the slow.
Take aim is maxed after shrapnel so you don't put yourself in danger while you are wrecking scrubs.
Use your Ult to 360 no scope noobs when they are low.
When to pick Sniper and how to use him
Pick Sniper when the enemy team is filled with low mobility heroes.
Don't pick Sniper against heroes that can jump on your face like Spirit Breaker, Storm Spirit, Phantom Assassin, and Axe.
You win if they don't focus you down or they can't jump on your face.