The simplest way is to just have the melody played down the octave. This gives you a strong sound. It’s not the most interesting, but it works

Notice that the interval between the top two voices is always a 3rd or 4th. This is ideal and correct for the traditional style. It is possible to have a major 2nd in this range but I try to avoid it; in the next octave up it may even sound like a mistake. A minor second should be totally avoided if you are going for a traditional sound

This is where you maintain the shape of the chord (same interval structure) and move it under the melody

It is handy to think of the 9th as replacing the tonic.

There is another option to consider when harmonizing the tonic chords. If you wish to create a really traditional sound, a la Glenn Miller, then use 6th chords instead of major 7th chords

There are a few ways to harmonize this F, but the most common method (and my default) is to use a ‘passing diminished.’