note by note, start by holding the first vocal note and finding it on the guitar. then do it for all of them and feel your way through it. then transpose it between the strings so it's in the best way to play it
Find the range of the singer, and just bust out some catchy riffs over your chords. Keep in mind breathing and rhythmic capabilities of the singer too
Whoever writes the vocal melody should know what notes they are singing, and then transpose to guitar.
Oh my bad, I thought you were writing the vocal melodies on guitar. Anyway, you should just sound them out. You know that though! Did expect there to be some secret that transposes it for you?