Step 15: Create A Clipping Mask

With the new layer selected, go up to the Layer menu at the top of the screen and choose Create Clipping Mask. You can also use the keyboard shortcut Ctrl+Alt+G (Win) / Command+Option+G (Mac). Either way "clips" the new layer to the layer below it. We can't see any difference in the image itself, but in the Layers palette, we can see that the new layer is now indented to the right which tells us that it's being clipped by the layer below it:

Step 16: Reload The Saved Selection

Let's bring back the selection we saved a moment ago, and we can do that easily using the keyboard shortcut Ctrl+Alt+4 (Win) / Command+Option+4 (Mac):

Step 17: Deselect The Opposite Square Selections

With the Rectangular Marquee Tool still selected, once again hold down your Alt (Win) / Option (Mac) key and drag around every other square selection to deselect it. This time though, you want to keep the square selections you deleted the first time we did this and deselect all the ones you kept, so that when you're done, your image looks like this:

Step 18: Copy The Selection To A New Layer

Click on the "Vertical Strips" layer at the top of the Layers palette to select it, then press Ctrl+J (Win) / Command+J (Mac) to copy the selected areas to a new layer. Again, we won't see anything happen to the image itself, but the Layers palette shows us a new layer named "Layer 2" above the "Vertical Strips" layer which contains our selected square areas:

Step 19: Create A Clipping Mask

Just as we did a moment ago, either go up to the Layer menu and choose Create Clipping Mask or press Ctrl+Alt+G (Win) / Command+Option+G (Mac) to create a clipping mask. We can now see the new layer indented to the right telling us that it is being clipped by the "Vertical Strips" layer below it: