I’m also planning a routine at the moment! I’ve done it twice before, but I still find it pretty hard.
I usually start by choosing a song (yay you’ve done that too!) and then I make a list of moves I would like to put in it. Depending on how far away the performance is I’ll include moves that I’ve almost got down but still need work. Then I work out basic combos for a couple of the moves. For example, invert -> V -> Crucifix -> handstand.
Then I usually print the lyrics of the music off and like chemgoddess said, look at certain points of the song and know what I want out of the performance. So, for example if the chorus is really fast I might do some spinny stuff there, so I work in the combos around the tempo of the song.
I studied music, but I don’t know much about it and I don’t dance to counts either, I literally just go off what ‘sounds / feels right’.
Then I just practice the combos with the different parts of the song, maybe freestyle bits and work out what feels best. Then practice, practice, practice!!
Like chemmy said, you want to include moves that are second nature to you. If I have a couple of months before the performance i will try to include harder moves because I have time to work on them, but otherwise only stick with what you know you can do without thinking.