I've posted this before, but this method is so simple compared to others that
I've seen on RFC, I thought I'd post it again. I haven't bought mayo in years.
An immersible stick blender (Braun) makes it simple.

In a glass jar (I use a peanut butter jar) put, in this order:

one room-temperature egg ( I put mind in a cup of warm water while I gather
other ingredients)
1 scant teaspoon of *fresh* lime juice
1 scant teaspoon of dry mustard
3/4 teaspoon salt
1 scant cup canola oil
*Without* turning it on, put stick blender all the way to the bottom of the
jar. Then turn it on, rock is from side to side as you pull it to the top. You
will have instant, delicious mayonnaise.
You can also do this in a food processor but you will probably have more
failures. (add the oil slowly as you blend). For me, it is so worth it to buy
a stick blender (Braun brand is best).
Refrigerate, of course, and it keeps for several days.
Enjoy!