If you are going down the neck of the guitar (starting from the pickups or sound hole and finishing at the nut) you would use FLATS, as shown below.

ON MOST ELECTRIC & ACOUSTIC GUITARS THERE WILL BE
DOTS OR MARKS ON THE FRETS THAT ARE SHOWN BELOW.

As you can see below, between all the notes that have a tone move between them there is an ACCIDENTAL (Sharp or Flat). B & C and E & F only have a semitone move between them so there is no such thing as a B sharp or a C flat or E sharp and F flat.