Place the egg yolk in a stainless steel bowl. Begin whisking in the grapeseed oil drop by drop, as slowly as you can bear. (I find having a second person dripping it in makes it easier.) Continue in this manner until the mixture begins to thicken. Once the mayo has emulsified, you can add the rest of the oil in a slow, steady stream (however, as a beginner, I still take it very slowly), whisking all the time. Arm getting tired? Try to murmur “jiggle-free upper arms… sleeveless sundresses…” to yourself. I find it helps.