When it comes to writing your own songs most people mess about with musical ideas until they fall in some kind of logical, musical structure. Most people will remember how they play their own songs, some may make a quick recording as a document and note as to how the song goes. What about writing the whole song down, chords, melody, rhythm and the whole structure? Why would someone want to do this? Once you can write down your melodies, chords and structure you will known exactly what goes in within your song. VEry useful for when you want to play your song with other people, or even just for yourself: It will make it easier to make changes in your songs, or in case something does not work, to see what it is that is not working.
When it comes to writing melodies guitarplayers can use tablature to write down the melodies: Find the notes you are singing on the guitar and write them down as you would do with for any guitartab. You can write down the rhythm underneath your tab, this so you will remember the feel of your melody.
Once you may have notated your song you may want to create different versions of your song, make variations in the movement of the chords, change the rhythm, maybe change the style completely. It is all up to you and because you can now see what is going on you are no longer searching for things in the dark.
Happy playing and hope to see you soon again.