nail down

/N EY1 L D AW1 N/
verb
  1. 1

    To attach with nails

    โ€œNail down the shelf, then paint it.โ€

  2. 2

    To make something (e.g. a decision or plan) firm or certain

    โ€œThey haven't nailed down their vacation plans yet.โ€

Source: https://en.wiktionary.org/wiki/nail%20down