I have been saving any avocado seeds I get and any glass jars that we have left from salsa!
All you do is take 3 toothpicks and stick them around the avocado so that it will set on the jar with half in the water.
Keep it filled.
It does take a while for a start to shoot out of it. But , when it does I always wait until the roots are pretty long before I replant it into a pot with potting soil.
A few years ago I had one that got pretty tall, but had set it outside on a hot and humid day, and a stray cat dug it all up.