After you obtained the amount of woad leaves you wanted, it's time to make the dye.
Making the dye is in a different location so buying many woad leaves are advisable.
In the town Draynor Village there is a witch located north of the library. The entrance to her house is inside the circle.
Right click on her and click on "Make-dyes". Three choices will show and you can choose which dye you wish to make, each require different ingredients. Click on the option in the middle and she will make your Blue Dye. Repeat this process until all your woad leaves and coins run out. (5 coins per dye). She will even do a weird emote near the cauldron.
If you wonder what the witch's name is its Aggie.
(30 woad leaves and 75 coins gives you 15 Blue Dye).