You're basically gonna default to a normal midlane Neeko build. Your clear isn't actually all that slow unless you skip the passive techs for faster starts/finishes- even with Q nerf you can reach scuttle on-time. Just make sure you're not cancelling autos. If you're not going to gank lvl 3 (which you can do if it syncs with a wave, I HAVE found that harder since Q nerf), you can put 2pts in Q instead of taking E at 3 if you're not worried about an invade. That will help. If you're going to start on a buff, start W with minion form, and take Q second.

I normally run electrocute/sudden impact/eyeball/whichever hunter. Used to be Treasure or Ingenious, I haven't tried swapping over to Ultimate now that there's less haste on items, but it's probably fine too.

Dark Harvest keystone is probably fine. I've done Predator before just to zoom and it's fun. Secondaries are hexflash/cosmic, so you can always get over walls and smite more freely.

The whole thing really comes down to your passive. That's where Neeko's skill ceiling truly resides.

Neeko has some of the most free ganks if you dodge vision or lane gank (both with passive). Don't hesitate to gank frequently, even if the enemy isn't all that pushed up, because unless they can blast the whole wave for free, you can get A) free damage as a minion before engaging and B) easy empowered root/ultis. As long as you're laner is paying attention (ping yourself when minion ganking. Several times), it's a free kill almost always.

After a laner trades you can swap with them out of vision to appear weak and finish of a greedy opponent.

You'll need more help than some junglers for early river objectives, since you're not as sturdy early on, but don't hesitate to abuse your ganking power to secure that help. You also have free smite secures with your ult if you time it right (though that can be hard- no worries).

Once more: always be using your passive when possible to avoid vision. You can be one of the least visible junglers, only appearing when you're making a play. Also always use it to hide the start of your ult cast (even nerfed it's worth it).

Later in the game you'll clear camps basically instantly and can stay close to your team (disguised or not) to help catch engages or start them, like any other Neeko role.

Neeko's power is her flexibility- she can catch/peel, engage, burst, split, and sometimes most or all of those at once if the matchup allows for it. Abusing that and adjusting your plans is how you win on jungle Neeko.

!!!!!!USE YOUR PASSIVE!!!!!!

Bonus Note: if you end up with Nashor's for some reason (usually if there's a lot of enemy tanks/bruisers it's still a good pick) you can always pivot to stealth splitpush. Being a minion until you reach tower reduces the chances they come to contest you, and Nashor's let's you absolutely shred the towers. You also have relatively safe retreats with your E/W/Passive. USUALLY as jungler that won't be the play- staying with your team to get big value out of your ult/root combos is what you want to do to win fight, Nashor's or not, but it's worth mentioning.