Start with a blank piece of music staff paper or even a sheet of notebook paper. If you don't intend to write melody notes for the lyrics, notebook paper is sufficient. Title the lead sheet with the title of your song. Include a key signature and time signature on a lead sheet written on music staff paper, especially if you will write lyric melody notes out.
Strum chords on a guitar or play them on a piano. Sing the lyrics and as you do, listen for when the chords change. The idea is to notate the chords as close to the actual place in the song lyric where the chord changes. Write down the first lyric line, then write the appropriate chord above the lyric.
Separate the music according to vocal lines, unless you're using music staff paper and writing individual notes above the lyrics to dictate melody. In that case, separate the lyrics into measures according to the time signature of the song. For notebook paper, leave enough room above each line of lyrics to ensure you can place the chords so they'll be easily read. Use an extra space to separate verses from the chorus or bridge.