Yea, I'm I'm with Linwood - a very brave first post and done well!
When reharmonizing a song, are there certain guidelines that you follow?
There are simple little tricks that I use.
I'll take a look at where I want a certain note in the melody to end harmonically on. Let's say I want the fifth note of the melody to have some kind of 'C' chord. I'll work backwards from there. The fourth note of the melody will have some kind of 'G' chord, the third note of the melody will have some kind of 'D' chord ... and so on. Sometimes you can force things through, sometimes not.
... or I'll approach the 'fifth' note of the melody (in this example) using chromatically ascending or descending chords. If I wanted a 'C' type chord on the fifth note of the melody, the fourth note will 'see' some kind of 'Db' chord ... and so on ..... or I'll approach it from a 1/2 step below.
... or I'll use the same exact voicing using parallel (block) chords. Each melody note will be, for example, the +11 of a chord and I'll use the same exact voicing for every melody note. (You can't do this for too long, say, four of five melody notes.)
Anytime I see a dominant 7th chord I break it up into a ii7 - V7. That alone doubles your chords and helps to move things along.
If I see a minor iv chord, I'll think a bVII chord ... and so on.
Before I reharmonize anything I simply play just the melody and listen.
These 'tricks' can be worked on systematically.