Ironing out a warp in a built up wing is easy. I find it easier to do with an iron, but in our hobby there are often 2 or 3 "best" ways to do most things.

You simply block the wing, usually around the middle, so it's steady, and with one hand you twist the wingtip so that the warp is twisted out. With a builtup wing, you'll see wrinkles in the covering. Iron 'em out while you hold the wingtip firm. You need to hold the tip firmly until all the bays are tight, top and bottom.

If the warp comes back a bit after you thought you were done, do it again, and twist a bit farther.

Pretty easy to do. And of course, you check to see how you've done with the technique as described in the post above.