Need To Be Squared Away on Modes

Hi,

I am new to music and trying to make sure I understand the concept of how modes work in the greater scope of guitar. I am looking for someone to look at what I think I understand and confirm or correct it.

A greatly simplified song...

1. We start with determining the Key, which provides us with the Scale.
2. Then we determine the type of tone we want (Mode) by choosing one of the degrees (I, ii, iii, IV, V, vi, vii) from our scale as our central tone/root chord. We play the root chord in a progression which provides the harmony and make sure the root is the dominant tone.
3. Then we overlay the harmony, with the notes from the original scale which makes the melody.

I am just looking for a high level confirmation that what I have defined above is correct.

Thanks