Once you have the strip complete, you then need to sew the triangle on the top.
Accuracy in terms of the amount hanging out each side, is not quite as important here (as long as it's an equal amount hanging out each side). But sewing quarter of an inch from the fabric edge still is. If your three-fabric strip is a bit wonky, then line the edge of the middle piece up with the edge of the big triangle and then sew a quarter of an inch from the edge of the big triangle.
I've been pinning a lot before sewing in this quilt, which I don't normally do, but it's been worthwhile in making sure things line up how I want them to.
See how the large green triangle is hanging out wider on each side? That's good! I know it looks wrong, but it gets trimmed off later to make an accurate 9" square.