Okay. So at custom install you must make sure you only delete the windows partitions. You should see the a 256GB partition (which is the D: drive) and the windows installation partitions will be broken into several partitions some MB in size and one other larger GB (the C: drive).
Your D: drive should show as New Volume. In this screenshot the windows partitions are arrowed. In your case they will all be shown as Drive 0. But you can delete any small size partitions.