I know what the Cauchy-Schwarz inequality is, but I don't know how to apply it.

My teacher mentioned that we should try "constructing" or changing the numbers so that the C-S inequality can be applied.

a and b are both real numbers, a+b = 1

using the Cauchy-Schwarz inequality, prove that 1/a + 1/b >=4

Any help is greatly appreciated!