Identify the key of the original tune. Identify any accidentals (notes whose pitch is not a member of the scale indicated by the key signature) or key changes.
Transpose the original key of the tune up a major second--one whole tone higher. If the original key is E-flat, the new key will be F. Write the new key signature at the beginning of the staff.
Transpose every note of the original tune up a major second. If the note in the original tune is a B, you will write a C-sharp. Transpose all accidentals up a major second. Transpose any key changes and insert the key change in the same measure, where it occurs in the original.
Review your work to make sure all note values are accurate. The transposed version is what the B-flat instrument will play, and the original is what the guitar will play.