Many musicians have no clue to write chord progressions to melodies. I had this issue in my early days of playing but after studying some basic music theory, it became second nature. I will show you a very simple process that you can learn in minutes and start writing dozens of chord progressions to your melodies. You will need some basic understanding of music theory in order to do this process.
Step 1. Pick a melody and find the parent scale it was written in
The first step is to pick a melody you would like to write a chord progress for and find the parent scale it was written in. If you do not know what a parent scale is, it is the main scale which is divided into modes and can not be found in other scales. The major scale is probably the most common of them all so we will use it as an example.
Step 2. Analyze the melody notes and find out their parent scale degrees
The next step is to analyze each note of your melody and find the correct parent scale degree. A parent scale degree is just the place of the note in the parent scale. For example, the note C in the C major scale would be the first degree, the note D would be the second etc. There are only seven degrees in the major scale. Find the right ones for your melody notes and move on to the next step.
Step 3. Decide which melody notes will be the chord tones
Now you have decide which notes of the melody will be chord tones or notes which belong to chords. Once you have decided what they are, you can move on to the next step and we will start experimenting with basic triads or three-note chords over those notes.
Step 4. Decide which of the three chord tones of a triad will be your selected melody notes
All triads in the major scale have three chord tones and they are the root, the third and the fifth. You have to decide which of those three chord tones are your melody notes. For example, I have a slow melody line of four notes in C major scale. The notes are C, D, F and G. I want to put a chord on each note. I decide that the note C will be the root of its chord, the note D will be the fifth of its chord, the note F will also be the fifth of its chord and the note G will be the third of its chord. Once you have done the same for your melody, move on to the last step.
Step 5. Use the parent scale degrees to find the missing chord tones
The last step is to use the parent scale degrees in order to find the two missing chord tones. In our example we have the note C as the root of its chord. The note C is the first degree of the parent scale which is C major so we need to find the third and the fifth of the chord. The way you find them is by adding the number two to each scale degree. The root is one so the third would be one plus two and the fifth would be three plus two. When we play the first, the third and the fifth degrees of the C major parent scale, we get a chord that has the notes C, E and G and that chord is C major.
The note D is the second degree of the parent scale. Because it is a fifth of its chord, you do the opposite to previous example to find the root and the third. You subtract two times by the number two. The missing lessons would be the seventh, which is the note B and the fifth which is the note G. When we play those notes together, we get the G major chord. I would use the same method for the F note because it is also the fifth of its chord.
The note G is the third of its chord so I would have to subtract by two and also add two to its scale degree to find the root and the fifth. Because it is the fifth degree of the parent scale, the missing degrees are the third, which is the note E and the seventh, which is the note B. That chord is E minor.
It is as simple as that! Practice writing as many different variations of chord progressions to the same melody as possible in order for this method to become second nature. Experiment with other chords along the basic triads. The more notes there are in a chord, the more chord choices you have. By using this method you will always write good chord progressions for every melody you have.