Keep your head down until you get Mystic Shot and Express Shot. These turn you into a menace to society for a good while with your alt fire, so once you get them you should try to look for as many fights as possible! Your alt fire will be your main source of damage early on, and you should always try to spam Splatter as soon as it's off cooldown to get the most value out of Compress Cooldown. Keep an eye on problematic players on the other team; counterbuy early and often. You're the ultimate opportunist. Take their Sinner's, pressure their Walkers, do as much as your goovement lets you get away with and nothing less. Puddle Punch and Goo Ball make you absurdly slippery. Spirit Snatch is busted - hitting two people with Spirit Snatch = +90 damage on Splatter, even ignoring shred - so practice syncing it with Splatter and Mystic Express. You can use your burst to annihilate squishies, Phantom Strike compliments this playstyle extremely well. During big teamfights though you should aim to backline. As far as passively maximizing damage goes, the most soul efficient item is Tankbuster (even very early on), followed by Spirit Burn. Mystic Reverb wins over Echo Shard in the long run, even ignoring the fact that it replicates Tankbuster damage. I personally prefer Echo Shard to secure kills on squishies, and it becomes much stronger with Transcendent Cooldown, although Mystic Reverb lets you imbue The Cube with Echo Shard later on and has lifesteal + slow. The entire Extra / Improved / Boundless Spirit line is generally worse than all of these and even scales worse, so it's really not worth it unless you're able to consistently hit multiple Splatter bounces on multiple targets or you're building Goo Ball (in both cases, I'd consider getting Greater Expansion).
Optional0 items
Splatter
The Cube
Puddle Punch
Goo Ball