Just that really 
Got the recipe off an American site and quite honestly, I thought it would be a disaster....but WOW, WOW, WOW. Absolutely amazing and way better than any shop bought stuff.
Scroll down for the video on 2 min mayonnaise.
If you don't want to click here is the recipe:
Equipment:
Stick blender
Pint glass
Ingredients:
250ml olive oil
1 room temp egg yolk
1 tbsp water
dollop of Dijon mustard
Method:
Stick it all in the pint glass and blend it