Spaced repetition brings a word back after time has passed. If you remember it, the next review can come later. If you forget it, the word can return sooner with a better clue.
Start with the reviews that are due. Add only a few new cards when the queue feels safe. If a card is unclear, fix the card instead of blaming yourself.
The schedule is a guide, not a promise. Sleep, stress, past knowledge, and word difficulty all matter. Use your mistakes to choose what to change next.