First, you get yourself a food processor. Place it on the counter in your kitchen, preferably near an electrical socket. Plug it in.
Fill it full of egg shells that you have collected over many months.
Turn it on.
Pulverize those shells. Turn them into little bits.
Cover the feeder tube so the dust doesn't invade your kitchen.
When you are done the egg shells should look like this (move your eyes a little to the left). Store in a tub with a lid. Sprinkle generously on little plants. Replenish if needed.
I have green bucket with a lid on a shelf in the kitchen, and I toss all our egg shells into it. It doesn't stink and it fills up every few months - it's sort of surprising how they add up. And every few months I grind them up and toss them into the garden. It works amazingly well.