Play walk in, tunnel after: approach burrowed, land W first, then use E to follow the flash/dash or leave through a tunnel you planted earlier, because spending tunnel to start makes Rek'Sai much easier to read and much worse at sticking.
Preload fury before you show, tag with burrowed Q, and look to spend full-fury E before using R through the return burst or tower shot, because Rek'Sai wins a lot of real fights by arriving with the bite already loaded and treating ult as protection as much as damage.
Rek'Sai gets paid by replaying the broken wall rather than by a pretty clear: recent data shows early gold swings her harder than early CS and late first-component games drop off fast, so once one lane or route cracks, keep hitting that same side while the enemy still has to guess your tunnels.
If the first cycle gives you nothing clean, stop trying to prove Rek'Sai is an early ganker and start building the next window. Full clear or near-full clear with fury managed, leave tunnels between camps and the lane you expect to revisit, and hover the side where the enemy jungler is most likely to start the fight for you. Rek'Sai is good at turning a countergank or overstep once she arrives burrowed with knockup and full-fury bite ready. On reset, path back to the lane that burned flash, used dash, or keeps trading long, and reuse the tunnel route instead of inventing a new play.
Rek'Sai generates Fury by Attacking and hitting with basic Abilities. She consumes this Fury while Burrowed to restore health.
Rek'Sai's next 3 basic attacks deal bonus Physical Damage to nearby enemies.While Burrowed, Rek'Sai launches a burst of void-charged earth that damages and reveals enemies hit.
Rek'Sai burrows into the ground, gaining new abilities and increased Move Speed. Her vision range is reduced and she cannot use basic attacks.While Burrowed, Rek'Sai may cast Unburrow to knock up and damage nearby enemies.
Rek'Sai bites her target, dealing physical damage, or bonus true damage if she has max Fury.While Burrowed, Rek'Sai creates a re-usable, long lasting tunnel. Enemies can destroy it by standing on top of either entrance.
Rek'Sai passively marks targets by damaging them. She can activate this ability to become briefly untargetable and lunge at a marked target for heavy damage based on their maximum health.