You don't. You ask for all the things that go with the outline - balance, rhythmn, straightness, suppleness and bend, and first and foremost that they are working forwards into a contact. Forget about the outline, and work on getting her on your aids, doing accurate movements and transitions.
You do have to be aware of your contact, the amount needed depends on your horse, but most importantly: however strong it is, it also needs to be soft so that your horse a) doesn't back off the bit, b) doesn't lean and try and run through the contact or c) goes round but learns to rely on a strong contact to hold him in the outline.
cvb (quoting Mark Rashid I think) says that you have to ask for softness (ie outline) with softness, and that's how I try to think of it.
I am always very conscious of my seat and legs (probably because I know that they behave strangely when given half a chance

) and my elbows, since when you bring your elbows back into your hips then they become softer and the weight from your contact can transmit down your arm into your seat and thus the horses back, connecting the front end with the back end. (sounds a bit abstract, but I can't really think of how else to explain what the importance of elbows is)
I tend to find that if I start off a schooling session trying to get a horse into an outline then I tend to meet with a lot more resistence than I do if I start with the intention of improving straightness, or really working on transitions etc, and the horse tends to find it more beneficial as well.
I will finish this ramble later, gtg, but I am heading somewhere with it I promise!