When I get ideas it's usually at the worst time, so I keep a notebook handy and note down the idea. Just in case I don't remember what it sounded like I write down a description of the atmosphere and mood I was in at the time that provoked me to feel what I felt, per say. That way, when the time is better I can see what I was thinking and I'll have some imagination to put to it. When I've got my guitar in hand I just sorta play around, practice what I already know, look for more ideas, etc. Though it's odd, all the ideas I've noted down and such I still haven't completely finished my first song that I made by myself. I've got one other piece that a friend of mine mostly wrote. Kinda upsets me, though somethings I give myself expectations that I should be far better than I am. Course that goes away when I go and learn something new and do something even more so complex. Thus just taking me one step closer to actually making something. All in time. If you feel comfortable with it, find a formula that you like to work with. In that respect there is no wrong way to write a song. If you're making songs then there's no issue. If you're not happy with your songs, then, well I recommend you rethink a lot of things.