Sacrificing the handmixer this evening. W00t.
To aerate the vat, whisk with an electric whisk or a manual one. Some people pour the liquid from one saucepan to another.
I usually whisk for 10 minutes with an electric whisk until the froth turns blue and then green again. Sometimes the froth remains blue however long you whisk but this does not usually affect the results. Once, in late September, I had almost no blue in the froth but, to my surprise, a large amount of woad pigment settled out.