I've been making my own for quite a while now and I can no longer stand the shop bought stuff.
If you have a stick blender you can make a batch in a matter of seconds. For some reason you can't use olive oil for this method (well you can but the end result will be metallic and bitter tasting - something about the speed of the blades affecting the molecules).
I usually use organic rapeseed oil which is fairly bland and makes a lovely mayo. I add garlic or herbs or whatever I fancy at the beginning too.
there are lots of how to's on You Tube - here is a link to one: