Is there a formula for writing guitar harmonies in thirds?

And if so, what is it. I have already written two melodies (one in E minor and the other in D minor). I know my way around E minor pretty well, but I'm not sure which harmonic notes to use (major thirds or minor thirds). I wrote down my melody in tab form and wrote a third above harmony for it strictly using notes from the E minor and D minor scales (I also want to know if only using notes from the scale is correct or incorrect as well please). I know that writing harmonies in fifths is easy because fifths are perfect, but I just can't quite figure out thirds. The formula I have right now for my D minor melody's third notes are (m=minor and Maj=major) [m][Maj][m][Maj][Maj][m][Maj][m][m][Maj]… I know it's kind of long but if there is anything you may need to ask me so that you can better answer my question please feel free. Any help is appreciated, thank you.