What way do you find most productive to start writing a song? Do you start with a song concept? Do you start with lyrics? Melody? Rhythm?
Any of those. If its melody it will be a single phrase. Developing this can be a trial: so many options! Rhythm will take care of itself most of the time, though may need a final arragement before its done.
If its Rhythm, this can be easy, laying something on top will be chords usually. Its just another route of assembling the structure. In both cases, the lyric will occur last.
If it is lyrics to kick off, I start with an invocation to the muse. If I get help, I get text down pretty quick. If not, I struggle endlessly. Either melody of rhythm can grow from there.
Do you have a common way of developing your original idea?
If adding something to a lyric, a phrase is helpful, as its an intuitive idea with elements of melody & time within in. These days I try to fingerpick a rhythm quite often, so melodic accompaniment is served by the chords. One common method I have for thinking of chord progression is to find a half tone interval common to all the possible chords in the pattern. This interval will feature in the final melody as well.
At what point do you start recording?
I dont.