I boil for half an hour, then lay them out overnight on a tray, then bake tray at 90 C for 30 minutes.
I use a nutribullet pro blender. It turns it into fairly fine white powder.
I don't remove the membrane because most of my eggshells have been out for a while and the membrane dries stuck. I do recommend rinsing inside of eggshell after cracking so you get a cleaner powder later.