First you have a statement like "3 teaspoons = 1 Tablespoon".
Now since these two things are equal, if we divided them together we would get one.
So basically we'll have the ratios
3 tsp./1 Tbsp. or 1 Tabspn./3 tsp.
and even though it may not look like it, both of these fracions equal 1.

Now which of the two ratios do we use? Well you want to convert from 1 unit of measurement to another. Whatever measurement you want change should be the same as the denominator and
whatever measurement you want to achieve should be the numerator.
Now just do a little multiplication

So if I wanted to convert 4 teaspoons. into Tablespoons,
I would use the ratio 1 Tabspn./3 tsp. or 1/3
4 * 1/3 = 4/1 * 1/3 = 4/3 = 1 1/3.
4 teaspoons = 1 1/3 Tablespoons.

Let's see this process again.

12 inches = 1 foot
Change 3 feet into inches
We want to get inches so 12 inches will be the numerator of our ratio.
We want to get rid of feet so 1 foot will be the denominator of our ratio.

3 feet = 3 feet * (12 inches/1 foot) 3 * 12/1 = 3/1 * 12/1 = 36/1 = 36
3 feet = 36 inches