•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.