How do i change open notes to flats or sharps on a guitar?

i'm taking guitar cles at my local college and i'm having problems making a flat out of an open note. eg. a single note like d2 is in the 2nd string 3rd fret. what about the open notes such as e1, b2, d4 etc. what fret would they go to? p.s. these are not chords, they're single notes