This is pretty obvious - but you need photoshop open, right?! lol.
Anyway, import your desired photo. I do this by dragging and dropping the image from a folder or the desktop straight onto the canvas in photoshop.
If you like to import photos through the menu system within photoshop - by all means, do it that way! The goal here is to get the image you desire to flip, inside of photoshop.
You Can Edit Your Image Before Or After You Flip It
If you haven't edited the photo in question yet, don't worry. You can edit it before or after you flip it. It's entirely up to you.
If I feel that flipping an image will improve the perspective and/or composition of an image - I normally flip it before I edit it.
I have edited it before flipping an image and found that I should have done it before and I would have edited it completely different (seeing as everything is now backward or upside down).
Again, you do your thing - no is no wrong way or answer here. Whatever works best for you and your needs.
Now We Will Transform The Image
Now that you have the image in photoshop and have it edited already or you're going to edit it later - you can now transform it.
Transforming in Photoshop involves many different ways to 'transform' an image. Flipping an image is one of them.
I will include two different and equally effective ways to flip your images either horizontally or vertically.
Go To Edit>Transform
In the photoshop menu - go to edit then go down to transform. The menu that pops up includes flip horizontally and flip vertically.
Click whichever one you desire. Your image will now be flipped.
Tip: If you find yourself not liking the flip, you can use the keyboard shortcut ctrl+z. This will undo the very last thing you just did. If you hit the shortcut too many times in a row by accident, you can use the keyboard shortcut ctrl+shift+z to redo actions you accidentally 'undid'.
Second Option: Ctrl+T
You also have the option to use the keyboard shortcut ctrl+T.
This will bring up the transform toolbar at the top of Photoshop. I look at this specifically - not because I use it to transform anything... but by seeing it, I know that I'm in transform mode.
Once I see this toolbar - I know I can right-click the image and the transform window will pop up.
If you look at the menu closely, you'll notice that right-clicking while in transform mode will bring up the exact same menu that's in edit>transform.
At the very bottom are two options for flipping vertical or horizontally.
It Doesn't Matter Which Method You Choose
Personally, I find the ctrl+T method to be faster than edit>transform. But regardless, either option will yield the exact same result which is flipping your image.
Whichever option is easier for you to execute and remember, is the one you should choose and use going forward.
That About Does It!
Pretty easy right?!
Believe me, photoshop can be overwhelming, and trying to remember everything can be pretty daunting. But much like anything else in life, the more you use something and find a method to repeat over and over again... you'll be amongst the other photoshop wizards that are out there!
You should now know how to flip images in photoshop, how to execute it, and more importantly, how to easily repeat it going forward.