Set the Orientation to Vertical and input 500px in the Position.

Then go back to View > New Guide and this time set the Position to 1100px.

Tip: If you want to erase a guide just select the Move Tool (Shortcut: V), click in the guide you want to erase and drag it out of the screen You should have something like this now:

Now with the Trees layer selected, use the Move Tool (Shortcut: V) to move the trees to the right until the end of the trees touches the left guide. Like this: · Hold Shift while using the Move Tool to move in only one axis

With the other two layers selected (Hold Ctrl/Cmd and click them), do the same thing but this time drag them to the left until the border reaches the right guide like this:

Hit Ctrl/Cmd+Alt/Opt+C to bring up the Canvas Size menu. Select pixels, and lower the Width to 600px.

And this is the result:

Go to Window > Animation to bring up the Animation panel.

This is the panel we’re going to use to animate our scene. It may seem a little bit confusing at first but it’s actually really simple. Let’s start with the trees, click in the little arrow on the left of the Trees to expand their animation panel, then click in the stopwatch of the Position, cause that’s what we’re going to animate now.

By clicking in the stopwatch there, we just told Photoshop where our Trees layer will start the animation (Position). Now we just have to grab that blue arrow in the timeline, drag it to the end of the animation, and tell Photoshop where we want the Trees layer to end. Click and drag the blue arrow to the end of the timeline (10:00f).

Select the Move Tool (Shortcut: V) and drag the Trees layer to the left. Drag it until the right side of our layer matches the right border of our scene (Remember to hold Shift while dragging so it only moves in one axis), like this:

If you did everything right, a keyframe will appear in your animation panel, and you can already see what your animation is looking like by pressing the Spacebar.

We’re going to use the exact same steps we used to animate the Trees layer to animate our two other layers, the only difference is that this time we’re going to animate both layers at once. Expand the animation panel of the Sky and the Stars Glow layers. Make sure that the blue arrow is at the beginning of our timeline, and click in the stopwatch of the Position in both of these layers.