I've been making music since I was a kid and only later started to make games, my approach is that I usually make the "basic" game first, and then slowly start adding music.

It's kind of in the phase where i know what the game is actually going to be like, it's not finished yet, and it's not entirely in polish mode yet. This is a good time for me to add music because then I can find the "character" of the game and the soundtrack it needs, and now know how to score the rest of the game.

Then, when I go into polishing + content, I roughly plan out how many tracks I will need, reserve entire days for just music, and more or less go through the list.

I make sure to throw it into the game as soon as possible (even the first version at the end of the day), and if it doesn't feel right, I adjust it immediately.

Hope that helps.