Grab your immersion (aka 'stick') blender and a wide-mouth pint-sized mason jar and lid. Add the following to the jar (in this order):
1 egg
3/4 to 1 cup oil (extra light tasting olive/coconut/avocado/etc - do NOT use EVOO!)
1/2 Tbsp lemon juice (or apple cider vinegar if you like it tart)
1/2 tsp prepared dijon mustard (or more to taste, mine's usually heaping) - you can use dry if you like
1/2 tsp salt or other seasoning (I use a 'season salt' blend) to taste.
Set the blender on top of the egg, then pulse on the lowest speed a few times and watch it emulsify! Once it starts to emulsify you can continuously blend and move the blender around until the oil is fully incorporated. You may need to stir it a bit to get the last of the oil to blend, but that's okay, it will set up in the fridge! Cover and refrigerate immediately. Keep it refrigerated as much as possible. Mine usually lasts a couple of weeks if I don't eat it all before then!