So, I've been looking at some of these lessons in songwriting for a while, and noticed there are almost none about composing the music for your songs. Mainly only lyrics. Here is what I do, and I write the music for songs quite often.
Where to start?
In my honest opinion, never try writing a song on an electric guitar. There are too many sonic capabilities while using one that could really distract you at the core of your writing. Its also much harder to get inspired, because an electric guitar is a much less intimate instrument. You will not feel as connected to your song, so it probably won't turn out as well. (There are some exceptions to this, i.e. writing a riff, writing a metal song, writing anything electronic (house, techno, dubstep), and if you really feel like using an electric, go ahead. I just prefer not to.
Where to go?
So you have your instrument. Next, just learn plenty of songs to be able to play on command. Its easiest to come up with an original chord progression if you know chords that sound decent in certain orders. Try to learn chords in the same key, or transpose the songs to the same key. Start trying to come up with combinations of these chord progressions, put the chords in different orders. Try some new things.
Where to pause?
Don't force your songs. If you are having trouble coming up with a second part of a song, write down or record what you have, and just do something else. It happens all too often to me that I will come up with another part for a song I'm writing while trying to write a different one.
Where to restart?
Restart whenever you feel inspired.
How to improve?
Try using different tunings? I'm not going to say which ones, because I really only know what one is called, and that is drop D. Come up with your own tunings. Just experiment really.
Next try using something that will change the lowest note on your guitar aka a capo. Though some might not seem like it, moving your song up a couple of frets on the fretboard can completely change the sound and feel of your song.
Another thing I like to do is alter fingerings of chords, leaving some strings open in a G or a C chord. Maybe moving the chord patterns up a fret or two. Anything that will make it sound different.
You can also do this with barre chords. Only bar the root and the octave of a bar chord. Just try something new.
Your song is finished whenever you are totally satisfied with it. If you are like me, you might then try multiple arrangements for it. I don't know.
Hope this helps. If it doesnt, oh well.