How do i make this into a skybox?

I’m wondering if there are any tutorials that tells me how i can create skyboxes from these kind of pictures?

Is that the original size or has the image been scaled down?
If I am right, I believe skybox sky textures need to be high resolution to look good.

It’s just a preview picture

I say go into photoshop and crop each photo.

How do i know which side is which?

top/bottom/left/right etc

Heres what I think it should be set up like:

Might need to do some rotating and shiz though!