Follow the setup steps here: https://sdq.st/setup

Complete setup and ensure you have a green connected dot in the top left corner of Sidequest when your device is connected.

Then in the Sidequest desktop app- click on the World icon along the top right row- type https://synthriderz.com in the custom search bar. The page will open in the SQ browser

Search for your songs- there is a little download icon on each song- if you click this, Sidequest will install it to the correct folder on your Quest.