When writing a song here are some things to keep in mind.
1-It all begins with an idea
2-Set down with your instrument of choice and flesh out the breadth of the song
3-Make a scratch track of your idea immediately, so as not to lose that vibe from the end product
4-Start to add instrumentation for a rough draft and brainstorming purposes
5-Marinate in the song, let it take shape, don’t rush it unless it is writing itself
6-Everything is subject to change at this point so don’t feel like you’re forced to go any one direction
7-Experiment with it, have fun
8-Assess the mood and build around that
9-Write down every idea, you don’t have to use them all but it’s better than forgetting something
10-Write out the BPM, the Chord Progressions, and lyrics and carry a copy with you, when inspiration hits you’ll be ready