S as a teenager. It is a lot safer than Al, and you can mix it cold.
Stick to solid propelants because it's simpler, safer and smaller. You don't want an explosion but something very quick burning.
Mix the two fine powders well.
You can mix them in a thick plastic bag or wide mouth bottle and shake until you see an even color in all the mix with no lumps in sight.
(You can make a paste with alcohol 100% but I never tried this.)
Then tap it in with wood or plastic. You don't want a spark! Nozzle reduction is 1/3 and not smaller. The mix is 3:1. Light with a filament, it is safer than a spark.
In those days all I had was a book but you can find a lot of info out there.
We switched to this after a (not too serious) accident with rocket candy.
Safety first allways, the rocket can go anywhere!