Julia - For me, (1) the most important thing for a cookie tutorial is great pictures. The writing should add detail, hints, and tips, but the pictures need to be good enough to show clearly what you need to do, as well as being visually enticing enough to make you want to actually do it!
(2) Then the writing needs to be clear and straightforward. You need to be really clear in your mind how you did something, and how to explain that to someone else.
There are certain conventions, and styles that work best, and that's something I'm still learning writing for CC. I never knew, though it now seems obvious, that equipment/ingredient lists should be in the order you use them for example. I see areas where Julia has edited what I've written and seen how it adds clarity, and I'm trying to retain those elements for future writing.
(3) Finally, though it's not essential for a good tutorial, is being able to convey something of yourself, to bring the tutorial to life. Lilaloa's tutorials are a great example of this, with her humour and snippets of home life mixed in. I find this the hardest bit, being naturally British and reticent! But I do try to add something authentic from me, to give it a "voice".