Alum can be found in the spice aisle at the grocery store. I don’t exactly know what it is (could this be another Fluther question??), but I remember from childhood experiments that if you put a dash on your tongue, it sort of “sucks up” the liquid in your mouth. It’s a white powder, sort of like baking powder.
Anyhow, the alum is critical because it makes the pickles crunchy. I think this is where many homemade pickle jobs go bad, frankly.
Small pickling cucumbers are best. I’ve never tried it with large ones- might want to cut them into spears or slices. Not sure if they’ll work or not.
To sterilize the jars & lids, you can do one of two things: first option is to run them through the dishwasher on a hot temperature. Second option is to lower them carefully into a huge pot of boiling water (big enough that you cover them entirely). Let them boil for 10–15 minutes. They should be sterile after that.
Be sure that you always use new canning lids. The rings are reusable as long as there’s no rust or disfiguration, but the lids aren’t meant to be used more than once- the seal dosen’t last.
If you’re feeling adventurous, you can throw some other vegetables in jars while you’re making the pickles. Onions, carrots, green tomatoes, etc.
Oh- and we usually wait about 6–8 weeks until opening the pickles (we always make ours on Labor Day. They’re usually done in time for Thanksgiving or Christmas).... but you might find that they’re done in 4–6 weeks. Open a jar and find out. If they’re not done, just keep the jar in the fridge and try them again in another week or two.