To blend the two images together using the layer mask, we don't use the Eraser Tool. In fact, while the Eraser Tool still has its place, you'll find yourself using it less and less as you become more comfortable with layer masks. Instead, we use Photoshop's Brush Tool, and with our layer mask filled with white as it currently is, which is making the entire layer visible, all we need to do is paint with black on the layer mask over any areas we want to hide. It's that simple!

To show you what I mean, I'm going to select my Brush Tool from the Tools palette:

I could also press B on my keyboard to quickly select it. Then, since we want to paint with black, we need to have black as our Foreground color, and by default, whenever you have a layer mask selected, Photoshop sets white as your Foreground color, with black as your Background color. To swap them so black becomes your Foreground color, simply press X on your keyboard. If I look in the color swatches near the bottom of my Tools palette, I can see now that black is my Foreground color:

I'm going to resize my brush to the same general size I used with the Eraser Tool by once again using the left and right bracket keys, and I want my brush to have nice, soft edges so I get smooth transitions between the areas of the layer that are visible and the areas that are hidden, and I can soften my brush edges by holding down Shift and pressing the left bracket key a few times. Then, with my layer mask selected (I know it's selected because the layer mask thumbnail has the white highlight border around it), I'm going to do basically the same thing I did with the Eraser Tool, except this time I'm painting with black on the layer mask over the areas I want to hide rather than erasing anything:

After spending a few more seconds painting away the areas I want to hide, here's my result, which looks pretty much the same as it did after I used the Eraser Tool:

If we look at the layer mask thumbnail in the Layers palette, we can see where I've painted with black, which are now the areas of the top image that are hidden from view:

Let's say I'm happy with this, and I think my client will also be happy with it, so just as before, I email the image to the client for their approval, save my Photoshop document, close out of Photoshop and shut down my computer. The client calls back a couple of hours later and says they like it but they want some of the bride's veil on the right brought back in. When I faced this situation after using the Eraser Tool, I was out of luck because I had deleted that part of the image and had no choice but to start all over again. This time though, I was smarter! I used a layer mask, which means that the entire image on the top layer is still there and all I need to do is make more of it visible!

I was able to hide parts of the layer initially by painting on the layer mask with black, so to bring back some of the image that's now hidden, all I need to do is press X on my keyboard to swap my Foreground and Background colors, which makes white my Foreground color, and then I can simply paint with white over the areas I want to bring back into view, again making sure that my layer mask, not the layer itself, is selected, otherwise I'll be painting directly on the photo itself, and I'm fairly certain the client wouldn't approve of that. I think I'll use a smaller brush this time with harder edges so there isn't such a large transition area between the two images, and I'll use the bride's veil, along with the tree trunk above her, as the dividing point between the two images, which will look more natural. As I paint with white on the layer mask, the areas I paint over that were hidden become visible once again:

If I make a mistake as I'm painting and accidentally show or hide the wrong area, all I need to do is press X to swap my Foreground and Background colors, paint over the mistake to undo it, then swap my Foreground and Background colors once again with X and continue on. And here, after a couple of minutes worth of work painting the veil and the tree trunk back into the image, is my final result:

Thanks to the layer mask, I didn't have to redo everything from scratch because nothing was deleted! The mask allowed me to hide parts of the layer without harming a single pixel, Not only does this give you a lot more flexibility, it also gives you a lot more confidence when working in Photoshop because nothing you do with a layer mask is permanent.

So, there we have it! That's the basics of how layer masks work in Photoshop!

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