There are a few different ways to teach subtraction, depending on the age and skill set of the learner.

Prerequisite skills: rote counting, receptive and expressive numbers, matches quantity to number and number to quantity, addition

Teaching subtraction by understanding quantity

Experiential Math Curriculum

Teaching Subtraction with Touch Math (You don’t need the actual Touch Math curriculum for this, just the pictures of the numbers)

Teaching Subtraction with Memorization/Fluency