In a blender, add everything but the oil and mix well for about 30 seconds. With the blender running continuously, pour in the oil VERY slowly at first - almost by drops, for the first 4 tablespoons of oil or so. Stop pouring and continue blending for 30 seconds, then start to add more oil in a THIN stream. Every minute or so stop pouring to ensure the oil is being incorporated.
Continue this way until you reach the desired thickness - this whole process takes around 10 minutes (you may not need to use all the remaining oil).
Stop the blender, and adjust the seasonings and, if the mayonnaise is very thick, process in drops of lemon juice or warm water to thin.
Keep the mayo in a sealed container, and it will keep for a week in the fridge.
Notes
Do not use light or extra virgin olive oil as it will cause mayo to separate when refrigerated.