Write with an instrument you're comfortable with. A piano or an acoustic guitar are ideal for songwriting because you can play melodies and harmony on both. Find a relaxing environment for writing and think about where you want to start. Sometimes you might have a lyric that helps kick off a song; other times, you may find a chord progression that sounds good. Start with what you have.
Strum a few chords to find a progression that sounds good. If you don't have words to the song yet, hum something over it to help form a melody. When you decide on the chords you want, write them in your notebook. Leave room for the words beneath the chords. When you decide on the function of the chords you have, write down in front of the progression whether they belong to a verse or chorus. If they belong to a verse, write the number of the verse as well.
Notate any musical fills or repeated music patterns that will be played throughout the song. You can use tablature or traditional music to write down passage. Write the musical passage where it will appear in the song and label clearly, such as "intro" or "fill 1." If parts will be combined, you can write something such as "fill 1 w/intro."
Add lyrics as they come to you. You don't need to have the lyrics completely written before you structure your song, but lyrics help set up a verse and chorus pattern for your song. If you don't have specific lyrics, write a basic idea for what you want to say.
Review what you've written to ensure the outline for your song is in the order you want it. If you've made scratches and notes to yourself on your original outline or you need to change something, write a fresh outline that's clean and ready to be written fully. A finished outline may look like this:
Intro pattern (write intro pattern out)
Verse 1 Cmajor Gmajor Dmajor
I was walking in the rain
Cmajor Gmajor Fmajor
On a lonesome highway . . .
Fill 1 (write the notes)
Continue writing the outline of the song until it's complete, then you will have a structure on hand to write the full song.