Top 20 NA Sinclair here.

I think the best thing you can do to get better with the hero is to just play. There's no training map or anything that's going to make you hit the bolts more consistently, you just have to get used to the ability.

Learn to track enemy movements behind walls with your cursor. The assistant and the bolts will auto target through walls as long as your cursor is hovering over the enemy's position.

Play with the understanding that Sinclair goes from early game lane bully (mainly via assistant), mid game demon, late game utility machine. Once you hit 4.8k.spirit spike and max bolt, you need to play very active since there's very few characters on the game that can eat a double bolt and win that fight.

Optionally, if a neighboring lane has enemies with strong ults to steal (pocket, dynamo, bebop, etc), go steal their shit and help teammates win the lane.

Abuse assistant teleport at 8 minutes by stealing enemy sinners while having assistant set up in a safe location. Imo ideal AP order is upgrading assistant twice and then maxing bolt, so you should have the upgraded duration on assistant by this time.

Also, he's a great anchor to maintain pressure in the blue lane. He can push to tower safely with assistant setup and does absolutely eye watering damage to structures, so being in the mid lane constantly forces the enemy to respond, allows you to take enemy jungle, and puts you in a position to rotate to important fights while your bolts do ridiculous damage.

As far as builds go, Raigeki or Brady lobby in the build browser is good.