Start with the note after which which the scale is named. For example, to build an E flat major scale, start with the note E flat. For an F sharp major scale, start on F sharp. This first note is known as the "tonic."
List the next six letter names of the notes to come after the tonic note. Start with the tonic note, and follow the musical alphabet to do this. For a D-flat major scale, your tonic will be D flat. The next letter names will be E, F, G, A, B, and C.
Identify the half-step pattern for the major scale. This pattern is the key to understanding the major scale. The half step intervals between the notes are as follows:
Between notes 1 and 2: two half steps
notes 2 and 3: two half steps
notes 3 and 4: one half step
notes 4 and 5: two half steps
notes 5 and 6: two half steps
notes 6 and 7: two half steps
notes 7 and 8: one half step
Identify the accidental for the second note of the scale, and apply it to the letter name of the next note. Two half-steps should span between the tonic and the second note of the scale, which determines the necessary accidental. If the tonic is D, the next note will be some sort of E. Because two half steps exist between the first and second notes of the major scale, the E must be an E natural. If the tonic is D sharp, the next note will be an F (the equivalent of an E sharp).
Repeat the previous step for the following notes of the scale, using the half step formula to identify the necessary accidental for each scale degree. The seventh note should be a half step lower than the tonic note, so the eighth note of the scale is the tonic note expressed an octave higher.
Reverse the order of the notes you have used once you reach the upper octave tonic to form the descending side of the scale. The descending major scale uses the same note names and accidentals that make up the ascending scale. In the descending scale, these pitches go in reverse order, following the upper octave tonic with the seventh scale degree, then the sixth, and so on until you reach the lower tonic.