To use them, just let your TA know. $400 per 100,000 points. You don't want to use more than that at once (unless this has been fixed recently) since you get less for your points. I believe, that if you have a joint account, you can each use 100,000 points. It used to be that you could 100,000 points more than once (e.g, at initial deposit and final payment) but I think Ngaire said they don't allow that anymore.

If you use goldpoints (lets use 100,000 for $400 as an example), they take the $400 off before certain of the discounts. Again, Ngaire is the expert but I believe it is before onboard, SSS, group, and back to back discounts. Assuming three 5% discounts, you really only get

400 x .95 x .95 x x.95 = $342.95 discount.

Now, if you use the goldpoints for a three day mini cruise which has none of these discounts, you get the full $400 benefit.