Homemade Mayo is easier than you can imagine with this foolproof technique (you must own a stick/immersion blender):

1 egg yolk (room temp!)
1/4- 1/2 tsp of salt (depending on your taste) 
Juice from half a lemon
1/2 tsp dijon mustard
2/3- 3/4 cup avocado oil (or any other good oil, avocado is my favorite tasting) 

Find a container that fits the head of your immersion blender snugly (this is important, if it's too wide around the head of the blender it won't work, you want no more than 1/8 inch room). Put the egg yolk, lemon juice, salt, and dijon in the bottom, in that order, then pour the oil on top. Let it settle, just for a couple seconds so the egg is at the bottom. Slowly put the blender down into the mixture, so it is suctioned at the bottom. Pulse it a few times, then hold the button about halfway down for 10 full seconds. You'll see the mayo forming at the bottom. At this point you can either start rocking the blender side to side, or slowly moving it up and down incorporating the rest of the oil.