how do I download spotify music onto my watch?

Are you subscribed to Spotify Premium?

If so, see the instructions in the "Download your playlists and leave your phone behind" section of https://newsroom.spotify.com/2021-05-21/enjoy-more-ways-than-ever-to-use-spotify-on-your-apple-watch-now-with-downloads/

designcon wrote:

I don't know what Spotify Premium is

That is easy to fix, you go to Spotify and read about it

https://www.spotify.com/us/premium/