To know which chords to match with which notes in a melody, first decide what key you want to work in. Then, find the I, IV, and V chords, because one or more of these 3 primary chords will have the melody note in it. For example, in the key of C, the I, IV, and V are the C chord, the F chord, and the G chord. So, if the melody note is G, you can play a G chord (G, B, D), since it has note G in it. Another option might be a C chord (C, E, G), which also contains G.

Each melody note can be a member of other chords in the key. However, choosing chords for a melody is also based on context, how each chord relates to the chord before and after it. When you use this method, it helps to narrow down the options and saves you time and energy.

Once you've mastered the I, IV, and V primary chords, you can start using more interesting chords, like extended chords (7ths, 9ths, etc.) and out-of-key chords, such as secondary dominants. For example, if the melody note is F, a different option than the basic F chord could be G7 (G, B, D, F), since it has F as the 7th. Another example is, that melody note G might be played over a secondary dominant V7/ii, that is, A7 (A, C#, E, G), which has G as the 7th.

In the video, I demonstrate this process of matching the basic chords to the melody notes, then matching the more advanced chords to the melody notes.

https://youtu.be/JIPfbJdM13Q