In my example I will use a small stack of 20 second, 120 second, and 420 second exposures all shot from my Celestron C8 SCT with a Hyperstar, operating at F/2.0 with a QHY-8 OSC camera. How many layers you use can vary from two exposures, to several. It depends on just how long your exposures are and how far you intend to stretch the data. For this exercise I will stretch the data to what I would consider a moderate degree, so only three layers will be used.
1. Open your stacks in Photoshop. We will work with the shortest exposure stack (20 second) first.
7. Feather your selection, again 30 to 50 should do it.
9. Apply Curves again, using a plot just like before.
11. This time we're going to repeat the process on the same layer as this one has much more exposure time and more data to be extracted. Select most of the bright nebulous area using Lasso or Magic Wand again. Feather the selection.
16. Pull up the third image stack, in this case the 420 second exposure stack, and paste the copied data into the image.
You should now have a fully stretched Orion Complex. This process can be expanded to include additional layers with longer and longer exposures. Below is what I was able to ultimately obtain using larger stacks of the exact same exposure lengths and combining them as above, and finishing up with other post processing techniques:
Q: "How do I know how much area to select for layering?"
A: This is where experimentation is advised as there's no magic answer. However, there is a really good way to get a feel for how much you should select and feather. Put two of the exposures side by side - i.e. the 20 second core shot, and the 120 second shot. Notice how much area is blown out in the longer shot. You need to select that much out of the shorter exposure and then some. When you feather you have to remember that it's a tapered effect on the edge of your selected data and that it will fade out gradually over the number of pixels you've selected. So if your selection is too small, and you feather, then move over to paste in as a new layer you may actually see the overexposed area bleeding through your new top layer causing discoloration, or a "ring" of sorts around your layer, making it very obvious what you did. You should aim to get not just the overexposed area selected in your short exposure layer, but you should try to expand that out an extra 10% or more to make sure you've got it covered. Feathering will take care of the blending for you.
Hope this helps.