Habit Stacking is a special form of Implementation Intention, binding the habit to an existing habit instead of time and place. This allows us to use the already strong habits to make the new ones easy to remember.
Formula
In essence habit stacking is just doing the new habit before/after an old one:
"After/Before {Current Habit}, I will {New Habit}
When choosing an existing habit to bid to, pay attention to it's circumstances, so that time and place are appropriate for the new habit