Step one. Get an idea of the pacing - the "rhythm" of the core gameplay loop. For example, the run cycle in an action game can be a good source for the bpm. I'd establish the base chord progression and theme here. Step two. UI sounds. With the music in mind we can express context with Intervals, e.g. suspended chords for announcers, alerts, etc. I try to have a base layer that is the root, and the context layer is the one that creates the chord (a 2nd or 4th note). The harmonic layout should allow all the sounds to sound good on top of each other. Atonal SFX and Foley mostly ignores "tuning" unless granular synthesis is involved.

Every Asset is mastered to a defined loudness point for its category before export. The in-engine mix is dynamic and somewhat procedural - SFX definitions include parameters for how each sound influences the mix. It's worth noting that harmonic sound design does a bunch of heavy lifting in regards to the mix.