Draw your clef. Remember that the clef helps you to later identify locations for all your notes. The treble clef's tip curls around the "G" line, while the bass clef curls around the higher "F" line. Notes on bass staffs are a full line (or space) down from their location on treble staffs.
Write your key signature if required. Key signatures denote what key your scale is in (if you're writing a scale, it will be in the key of the first note of that scale). They are the sharp or flat symbols directly after the clef, and denote that every note of that type (sharing that line or space) is altered by one semitone (a half-step), unless a natural symbol cancels it out later. The C Major scale has no sharps or flats, nor does its relative minor scale, A Minor.
Note that when writing scales as a theory exercise, you may not be asked to include the key signature and instead be asked to write in the sharp or flat next to the specific notes.
Write in your tonic note. The tonic note is the starting point of your scale. If you scale is ascending, start as low on the staff as you can. For example, if you are writing a C Major scale in a treble staff, you would actually have to start below the five lines of the staff. Draw a short line below the bottom line and draw a note on this line. This notation refers to "middle C" on standard pianos.
Write the rest of your scale. If you're writing a scale that is new to you, you can figure out the rest, including sharps or flats, by following the basic structure all major scales follow in their construction: T-T-S-T-T-T-S, where T=Tone, or full step up, and S=Semitone, or half step. For example, if you were writing a G Major scale, you would start with G on the second line of the staff, followed by A one tone above, then B, and then C, which on a piano is a semitone above B, as there is no black key between them. The next note would be another tone up, bringing you to D, followed by E and F, each one tone up, and finally F#, a semitone above F.
End your scale with the higher tonic note. In a G Major scale written on a treble staff, this note would sit just above the top line. In the C Major scale from an earlier example, the high C sits in the third space on the staff.