There are a few ways but the most popular way is by using conversion factors.
For instance, lets look at inches to feet. lets say I have 24 inches and I want to know how many feet 24 inches makes up.
We know that one foot = 12 inches. We will use this as our conversion factor but written it a little differently. We can write it as 1ft / 12 in or 12in / 1ft
We can setup our problem using the conversion factor like the following: