Greg Howlett is a Christian concert pianist and recording artist. I found him while searching for free sheet music online in 2010 or 2011 (I think).
I consider Greg’s lessons and blog posts on music as gems for self-taught hymn playing pianists like me. I have always wondered how musicians create good sounding music, and here he is teaching, using simple and easy to understand words, how to make beautiful music.
I have watched many of his free video lessons and blog posts on reharmonization since the time I found his blog, and I started experimenting with the reharmonization ideas I learned, but I stopped about five years ago when I started working away from home where I do not have an access to a piano. 
But I’m going to start again this time. I hope… 
Listed below are some of the reharmonization ideas I got from him.
1. A ii chord substitues for a V chord (and a IV chord).
2. A iii chord substitutes for a I chord.
3. A vi chord substitues for a I chord or a IV chord.
- The first three above are from his blog post “Minor chord substitutions”. He also has videos on this: “Minor chord substitutions video demonstration (Part 1)” and “Minor chord substitutions video demonstration (Part 2)”
4. Change V-I cadences into ii-V-I cadences.
- From “Reharmonization video: I Surrender All”; also in “Video Lesson: What a Friend We Have In Jesus (Part 2)”
5. Insert a minor 5 (v) and dominant 1 (I7) in front of a 4 (IV) chord
6. A dominant chord (V7) usually resolves down a fifth (to a I chord) or sometimes a half step.
7. Very often, if a progression is moving with the circle of fifths, you can change one of the normal minor chords to major (making it a secondary dominant).
8. Add a flat 9th to secondary dominants if resolving to a minor chord.
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Ideas 6 to 7 are from “Secondary dominants”
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Another video on secondary dominants: “Demonstration video on secondary dominants”
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In a recent post, “Choosing Between the 9th and b9th”, he also said that it is also sometimes okay to use the flat 9th to secandary dominants even when not resolving to a minor chord.
9. minor 7 chords likes to resolve down a fifth or in diatonic steps.
10. You can insert a diminished chord betweet 4 and 5 chords.
11. You can substitue a #4 dim - minor4 6th - minor 3 - flat3 dim - ii7 - V7 -I for a V-I cadence.
12. Which color note belong to each chord?
Major/Major 7th
9, #11, 13 (9 is the most common)
Minor 7th
9, 11, 13 (9 is the most common)
Dominant
b9, 9, #9, #11, b13, 13 (All are widely used. However, you are most likely to use b9, 9, b13, and 13.)
13. Tritone substitution - ex. substitute a F#7 for a C7
- Learn about that here
14. Substitute I - IVm6 - iii for I - IV - I
- I cannot remember anymore where I got this but here is an article that gives an explanation about substituting a a minor IV for a IV
“Any time you see a IV chord, a minor IV will likely sound great. The big exception? If the melody note is the third of the IV, it will sound horrible.”
- And here’s another one: “Piano tip – Using the minor iv chord”
Here is a sample of his free video lessons: “Relating harmony to melody”
