Harmonization

Music basically consists of the Bass line, the Harmony and the Melody. An interval represents the distance between two notes. For example, in the key of C Major, the interval of a C note to a D note is a second; C note to an E note is a third and so on.

In harmonization, you have to know the key signatures. This is because you need to Sharp a note sometimes. For example in the key of G Major, all F notes will be sharped even if the intervals say otherwise.

There are many ways to harmonize a melody:

Harmonization in thirds : To do this, we add notes that are a third below the melody note. For example, an G note gets accompanied by a E note.

We can change our fingerings for the chords so as to accommodate the bassline, melody and harmony all together.
Harmonization in sixths : To do this, we add notes that are a sixth below the melody note. For example, an G note gets accompanied by a B note.
Harmonization in fourths : To do this, we add notes that are a fourth below the melody note.

In the above case, harmonization in fourths sounds pretty bad. But it can work well in some melodies, like the one given below:

None of these harmonies will work for every single note in every scenario. Fourths, especially, are not traditionally meant to be used to harmonize every note of a melody. In fact, fourths moving in parallel motion was mostly forbidden in classical harmony.

There's a time and a place to use thirds sixth and fourths, and you don't have to commit to using just one throughout an entire melody. You can use them interchangeably based on what sounds best for any given scenario. If something does sound off it usually means that the harmony note is conflicting with a chord or bass note; So use your ears and experiment with different harmonies.

Different harmonies can change the whole feeling of the song. It may make a happy song sound sad, and vice versa. See this video for an example.