To form a Harmonic Minor Scales you should raise the 7th degree of the scale by a semitone.

(Remember the lowest note of the scale is always 1 and the highest note is 8.)

e.g A Harmonic Minor

This scale has no sharps or flats in the key signature.

In the harmonic form it would have an accidental of G sharp (--) ascending and descending.

A, B, C, D, E, F, G--, A

This will give you the semitones between notes 2 and 3; 5 and 6 ; 7 and 8