Dying with woad is actually a lot more difficult than it is made out to be here.
A full set of instructions is available here:
Woad
Suffice to say it involves steeping the woad for a long time, then making the solution alkali (ammonia/urine added). Then it needs to oxidise while hot (beaten over heat with say an egg whisk for a /long/ time until it starts to go slightly blue). Leave to sediment out the blue indigo dye, and rnse and repeat this settling several times. Leave this to dry out until a blue powder remains. This is your dye.
(Now you just have to make it up into a solution, dye your material, add mordants etc
)
As to the references to the early peoples of Britain using woad as a body paint, this is probably a big myth. The extraction of woad was a difficult and time-consuming process, and was used as a dye-stuff only for the upper classes in society. Where woad was used, and continued to be used right up into the middle ages, was as an antiseptic ointment for wounds in battle (woad itself is an antiseptic astringent plant, and when mixed with urine would form a sterile dressing - which would eventually turn blue). It is entirely possible that it was seen on the fallen bodies after battles and assuemd to be war paint.
This all comes from a casual quote by Julius Caesar - "Omnes vero se Britanni vitro inficiunt, quod caeruleum efficit colorem." - All of the British dye themselves with a glaze, which gives them a blue colouring.
This has been assumed by many to be woad, but is more likely to be linked to the Pictish tradition on tattooing (in the same way as the Maori). this has been verified by some of the bog bodies which have been discovered with blue-ish tattoos caused by using a mixture of carbon and copper/iron as the pigment.
Right - enough from me!