Write your song lyrics on notebook paper one line at a time. Leave a blank line above the song lyric where you will place your chord names. There is no set way you have to write. Some composers have a lyric first, some have a progression of chords that sound good. Regardless of your composition technique, you want to wait until the words are written down so you can use them as context for placing your chords.
Play the chords when you've written a line of words. Listen for the location in your lyrics where the chords change. Write the chord letters in the blank line above the lyrics, as close as possible to the exact location where the chord changes.
Place a blank line between your verses and choruses. Be sure to include the blank line for the chords as well. Play through your song often as you write, singing the words as you play the chords. If the chord changes aren't notated exactly where you want them and that results in rough transitions, relocate the chords to make it smoother.