I had trouble with that, always going back to the "SohCahToa" mnemonic. Nowadays, I always use the unit circle. Cos is abs[cissa] (horizontal), Sin is ordinate (vertical). Rotate the unit circle (or your drawing) to align as needed, scale with the radius: https://en.wikipedia.org/wiki/Unit_circle

You can derive that back from the Pythagorean theorem, but only if you have the sine and cosine definition in mind (SohCahToa).