Lets *Goo*

Early: Make sure you buy items that help you survive and win laning phase, consider your matchup and build accordingly

Early - Mid transition: Adapt to the development of the game, adjust resistances, utility and damage
Start to make upgrades.

Late:_Viscous is a very flexible character (pun intended)
Adapt your build to the requirements of your team
You can therefore choose between leaning into a damaging and waveclear build or focus more on support/utility

I strongly suggest buying Rescue Beam since it complements The Cube exceptionally well.

In the early stages of the game, Viscous wants to be on top of his opponents
Puddle punch allows for disruption of enemy movement and getting them into melee range
In a winning lane Splatter can be used to poke oponents hiding behind cover, in a loosing lane Monster Rounds can still allow for pressure
Generally buying Melee Lifesteal and Spirit Strike early makes Viscous a pretty opressive laner
Healing Rite is also very recommended in some matchups.

During the midgame its generally usefull to decide if your team needs damage or support
Both playstyles have very good waveclear
For a damage focused build get Rapid Recharge and Soul Shredder ASAP, then add the usual Spirit and Weapon items
For a more supportive playstlye buy Rescue Beam for its synnergy with The Cube and general healing and movementspeed for roams.

In the lategame Viscous nukes minionwaves
Use this waveclear to help your teammates create pressure
When playing a damage focused build splitpushing is an option, disengaging with Goo Ball if required (while hoping your teammates do something elsewhere on the map)
For the more supportive Build you may want to keep an eye on your carries, peeling them with Puddle Punch and protecting them with The Cube
Note that Rescue Beam can be used while your teammate is in The Cube, therefor allowing you to pull them away from subsequent danger.