I usually search for playlist with a similar vibe I'm going for to see what they've added and get inspo. Look at related artists to the ones already in the playlist, and create a radio station out of the songs to automatically get recommended similar songs! Just click around and listen and explore!! You can also look at the genres of the songs that have your vibe and explore those (everynoise is a cool website to explore genres with).
Most of all try to let it evolve, you will come across songs eventually that'll fit perfectly and you can add them as they come. Let your playlists grow over months and years, it doesn't have to be all done in one go!! :)
