I usually start with the intent of the music, and slowly work from there. For example, very early on I already planned my premise of a dreamy like world, juxtaposed with a much more solemn reality. From there, I would pick the genre, the two emotions I wanted to evoke in different tracks, the recurring motifs, and start gathering ideas. Sometimes I'll try to find other songs that have the vibe I pictured, and then I'd try to emulate that sort of style. I usually make the melody first, then the chords, embellishments, tweaks and polishing come later. Mostly cos I like to write tracks with heavy motif referencing (thanks toby fox), so I tend to come up with that and then write the other parts around it.
In retrospect, planning out what songs I needed, as opposed to writing the pieces as the inspiration came, would've benefitted me in this jam, especially since I was really short on time haha. My original plan was to make at least 5 tracks, but due to a lack of organisation, I only managed to write 3. One of my main regrets is that I didn't have the time to write a "main menu" theme, which would've helped to tie my tracks together. Not sure if that is out of topic, but putting it here anyway cos I think when it comes to making an OST, planning the skeleton out would make the workflow more streamlined, and wanted to share that thought.