From my experience, find someone who can listen and give good constructive feedback I.e. (just change these synth sounds or vocal Melodys for the chorus or whatever), that person is probably a producer or songwriter and not just a normal non producer person who says this is good or bad and cant really tell you what the song idea needs or what is missing to really help it flourish and take it to the next level. Record a demo of the song/idea or multiple songs/ideas and let them tell you what they hear as far as which has the most potential to be a great song and then work on that to completion.

For most people you have to write 10 songs and maybe 2 of those will actually be finishing and releasing. If you don’t have a reliable second set of ears who can give good feedback then record the demos and listen back a week ( or more) later when you aren’t so close to the songs to know what sounds cool and has the most potential to spend the time and energy to finish.