At least for this concept album, I tried to imagine the experience of players entering a biome, exploring the space, and finding themselves in a boss fight by splitting the composing into three parts:
1. Establish an evocative setting through sound design, ambience, and instrumentation.
2. Add main melodic lines to reflect the mood/atmosphere the player character has found themself in.
3. Twist both of the above when facing combat by adding electric guitar and heavier drums.
The meat and potatoes of the actual composing was just finding instruments that reflected the image I had in development (in the case of Strange Tides, mandolin and synthetic waves to evoke surging waves or stacks of transient pitched percussion to evoke a deep cavern), expressing a melody that fit, then of course the BOSS FIGHT TIME!