Kennen storm of shuriken do you have to targets before or after using zaun warrens

Before — Kennen's target is locked at finalization, which happens before Zaun Warrens' discard-draw resolves.

When Kennen conquers Zaun Warrens, both "When I conquer" and "When you conquer here, discard 1, then draw 1" trigger simultaneously (Rule 383.3.d). You order them on the chain as you like, but both triggers finalize before either resolves (Rule 337.1–337.4), and a triggered ability's target (the spell in your trash) is chosen at that finalization (Rule 402.2). Zaun Warrens' discard only happens at resolution — after Kennen's choice is already locked in.

So you must pick a spell that is already in your trash (e.g., from Kennen's own Burn 2 on play or earlier milling). A spell discarded by Zaun Warrens in the same conquer arrives too late to be chosen for Kennen's Flow.