Put all the ingredients except the oil into a blender and start blending. Trickle in the oil through the spout as it blends until it reaches the consistency you are most happy with (we like ours quite smooth but not too liquidy, but some people prefer it more bitty or thinner/thicker).
Recipe Notes
A little goes a long way with pesto. I tend to only use about 2-3 tsp per person in a recipe, sometimes less if I want it not to be the predominant flavour.