If I don't already have something in mind,
ill usually go through the OST of other games and try to find instruments on my Casio CT-X700 keyboard that sound similar.
I try to stick to my Casio's pool of 600 instruments.
Most of ARSONATE's music was re-used from my previous project, ">HAL_4", which was inspired by Portal.
So I tried making my own "Portal music".
Sometimes i have some stockpiled scraps of music that I couldn't figure out what to do with.
And I just use those.
For "Do You Feel Your Regrets Mounting" in particular,
I bought my first guitar amp and wanted to use it for something.