The easiest way to think of a harmony (in my badic way of understanding it) is playing the same melody a set distance away from the original melody. So for example if your melody was a super simple three nothe thing from D to F back to D, you could write a harmony in Thirds. Going from F to A back to F, and you’ve “harmonized” in thirds. This works with pretty much any interval depending on the sound you want. Experiment with different intervals to find the sound you want!
Make sure you reply with any questions i might be able to help with!