I teach this. Easiest way is with a proportion-once I show a student to properly set up a proportion, and solve for it (or create a formula using one), such a conversion is easy and virtually foolproof: no agonizing over “should I multiply or should I divide?” Proportions also have the advantage of being applicable to a wide variety of applications.
Make sure the same unit is on both numerators, and both denominators. In this example we’ll put feet on top:
------- = --------
Now putting the numbers in, with the base conversion numbers on the left, the numbers you are working with on the right:
1 Foot…2 Feet
----------- = ----------
12 Inches…X Inches
In a simple example like this it then becomes obvious that if you double the feet you also have to double the inches, hence the answer is 24 inches. In something more complicated you just cross multiply.