First, you need to get your Spotify username. Click the dropdown menu next to your profile picture on the top right of your home page and click on 'Account'. A new web page will open, and then copy/paste your Username into the 'User ID' inputs on the left side of this site.

Your 'Liked Songs' Spotify playlist is private, so we need to create a public playlist called 'publicLiked' that we can access to generate your party playlist. To do this, in the Desktop App, go to your 'Liked Songs' playlist, click the top song, then click 'Ctrl + A' (Cmd on Mac) and all your songs should now be selected. Now, right click any song, then click 'Add to Playlist', then 'New Playlist'. Finally, you should see 'New Playlist' in your PLAYLISTS on the left, rename that playlist to 'publicLiked' (without the quotes). And your done.