Basic painting. Have a light source by the example you have it is straight from the top.So all the tops of the letter get a highlight and all the bottoms gets a shadow.
Same thing for sunk in lettering but you reverse the highlight and shadows.Kind of like what i did here on the stone and wood .
That is it. You first paint your metal effect or as you seen in the mail box I did stone or wood .
Than place you letter layout down and lay in your letter. after the letters are on you than hit the bottoms with a darker shadow color of the metal or an over reduced black and build it slowly. You highlight hit with white and just a touch of the metal color..
You can either cut all your letter from tape or is you have a vinyl cutter you can use that.