To do the harmony... you need to know the key/chord progression.
If you don't have it, and can't analyze the melody in your head to get it, you're stuck.
Your's is starting in D. Most of the ones you're going to find are in C because the song was originally done in C. You might consider starting in C instead. Maybe something like this:
Or.. I guess if you want to stick to D, you can move all the chords up a step from the ones given.
I'd suggest learning the chorded version first, and then adding or pulling out the melody line from the chords, as it might be easier than the other way... at least it's easier for me. My problem with going from melody and adding chord, is that you're looking at the melody notes, without the rest of the structure.
Like.. I'd start with a barred D because the original started in a major chord, and that leaves my ring and middle free to get the next melody bits while still holding the treble part of the d
5
2
2
2
But like.. Ink started with an Em, and then adds the melody on the A string with the pinky. That will work fine as long as anyone you're playing with is also in minor or if you're singing the melody. But the original was in major and if your other players aren't using the same progression you can sound off as a group.
This is actually a hard song to start thinking about this stuff..... Just saying. For learning to do it, you might want to start with something simpler.