Then go to Image > Canvas Size, check the relative box and type 40 in width and height so we will have canvas extended by 20 pixels each side.

Now fill the whole document with white. Then create a rectangular selection within a guides. Copy your map to clipboard and then go to Edit > Paste Into so you will have fastly map inside a mask. If it is needed, use Edit > Transform > Scale to make it fit.

We are going to make a map based on four layers so all the transformations will be super easy. Start with selection showed below, go to Edit > Copy Merged and paste the clipboard into our layout.

Another selection, again- copy merged and paste to the layout.

Yet another, I suppose you already know how the last gonna looks like, but I will show it anyway just in case.

Turn this messy stack of layers

Into an organized stripe:

Make a group of those four layers. Select them in layer's window and hit CTRL+G. After that, select background and then with CONTROL select our newly created group...

... pick up Move Tool (V) and choose Align horizontal centers.

From now on our map should be perfectly centered on the viewport. Now we will make some simple transformations. Select first layer and go to Edit > Transform > Skew. Change the anchor point location and then type negative value into Vertical Skew.

Make the same with second, third and last layer. The stripe should be continuous, so remember to change anchor point and vertical skew accordingly to the situation, like here for second layer:

As you can see anchor point is on a right edge now and a skew has positive value. I have zoomed in image in the corner when two layers met to choose the right value for vertical skew. If the layers has the same dimensions, the value would not change, but here inner layers are smaller than the first and last. Here how it looks like for third and fourth. I hope you would already do that without these reference images.

Now select all four layers (but don't merge them) and go to Edit > Transform > Perspective. Make something like this.