Unfortunately, this doesn't work quite so well if you have the singing voice of the aforementioned sick cow. The joke is especially bad if the person you are serenading is a veterinarian's daughter (true story.)

I recommend wearing typical spanish gear, an outrageous sombrero and and light guitar.

LEARN TO PLAY BEFORE YOU ARRIVE.
Failing this get a backup band to serenade her while you sing the basic melody. It might be a good idea to give her *some* warning about this otherwise it may not go well.

Also, and this is vital, make sure she is in...

(alas another true story)