You get five turns. Each turn, tap or drag tiles from your rack onto the board to make the highest-scoring word you can, then Submit. Drag tiles around your rack to rearrange them.
Every turn is marked out of 100 on how close you came to the best word available — but the marking is steep, so half the best play is worth 25, not 50. A word that isn't in the dictionary scores nothing and still costs you the turn and the tiles.
Stuck? New rack trades all seven tiles for fresh ones. Three a game, and it costs you nothing.
Five turns, 100 each — 500 if you find the best word every time.