How to play Rek'Sai in 2019?

Usually if a champ has strong autos, they run Conqs or Lethal Tempo.

A conqs build is usually with the flat AD jungle item a lethal tempo build is with the AS jungle item

But you could say Reksai is very very close to Vi (she runs Electrocute, AD jungle item, Triforce then Steraks)

Wait for the buffs.

but more seriously though, rek'sai is a bruiser so build damage early, then transition into more defensive picks like steraks, GA, or even tank items.

Get your level 3, and gank as much as you can

Most rek'sai's go hail of blades for faster fury for bite.

Honestly just watch this for basic gameplay. https://www.youtube.com/watch?v=UOQTuTTCwiU It's a few months old so things have changed a little, but overall still a decent guide.

Runes go Hail of blades, sudden impact, whatever you want for 3rd, relentless hunter secondary precision with triumph and alacrity attack speed, adaptive force, and armor

build Warrior > BC > bruiser items (sterak's, GA, maw, etc)

you pick a different jungler

She's garbage right now unfortunately :/ I wish they would just fix this champion.