Watch the Melodies Using Secondary Dominants online guitar lesson by Matthieu Brandt from Songwriting on Guitar
Secondary dominants are chords that have a V-I relationship to another chord. In an Ionian major scale you can find these chords on the 2nd, 3rd and 6th degree of the scale. These chords will have a natural tendency to resolve to a chord thats 5 half notes up. To strenghten this progression you can alter these minor chords into major chords. When you do this, you need to take this alteration into account and adjust the melody accordingly.