I create a playlist to fit a mood (for example, ambient electronic music with a good beat and no words), and I seed it with at least 10 songs.
Then, I go down to the "Recommended Songs" section and I just play whatever it recommends.
If I don't like a song, I hit the skip button on my keyboard. The vast majority of what it recommends and plays is tolerable but not great.
Every now and then (maybe one out of every 20-30 songs) I will discover a shiny new gem of a song that really resonates with me. So I drag that song up and add it to my slowly growing playlist.
This has the effect of subtly changing the recommendation algorithm for that playlist over time.
Eventually I end up with a playlist with about 100 songs on it, and recommendations that at least roughly align with the mood.
I tend to get bored of songs pretty quickly though, so when a playlist gets too stale, I will prune the old and tired songs from it.
This approach has worked pretty well for me so far, but I really wish that there was a way to explicitly tell Spotify that I "dislike" a song and to never recommend it to me again.