Put something your child wants where they can see it. Set a timer for ten seconds and explain that when it rings, it is theirs.
Sit with them and wait together. Talk, sing, count — filling the wait out loud is what makes it survivable.
When it rings, hand it over immediately and with no conditions. Add a few seconds only after a wait has gone well twice.
Waiting is a skill built by repetition at a length the child can actually manage, not by being told to be patient. Sitting through the wait alongside them turns self-control into something shared rather than something demanded.