Pick something to do
Decide on a thing you want to give 15 minutes of your time, energy or attention to. This can be something that brings you joy (latte heart art); a task you want to complete (responding to emails); or a small step to something bigger (Je veux apprendre le Francais).
Set timer for 15 minutes
Take your phone, computer, Apple Watch, kitchen/egg timer, whatever works for you - set a timer for 15 minutes. Then forget about the time constraint. The clock is there to ensure you hold yourself accountable to giving a full 15 minutes - no more and no less.
do that thing!
Give it 15 minutes of full focus and effort. You picked this thing as something important to you for one reason or another (makes you happy, contributes to your productivity or helps you feel accomplished); free yourself from distractions and go all in for 15 minutes.