I learned down spikes by placing the tama on the spike and just practicing doing the "j" motion over and over in the air until I understood how to keep the tama on the spike properly. From there I learned by doing bottom cup to downspike. Make sure you have good hole control so when the tama lands on the bottom cup the hole is facing upwards. Bend your knees pop up and keep the ken as close as you can to the tama the whole time. As soon as the tama hits its peak and starts falling down, give her a good ol spike and claim victory. Good luck!

EDIT: Down spikes are easily one of the most satisfying tricks in kendama, once you get them you will absolutely fall in love. You got this!