Step 1: Opening the photo & Erasing the background
Once you've opened Photoshop, go to File, Open and find your file on the computer, once you've done that click open. Now that you have your photo open, let's start by getting rid of the white background. Use the magic wand and click on the white background, once the background is selected, hit the delete button on your keyboard. Now that you deleted the background, open the picture of the person you wish to photoshop onto Yoshi.
Step 2: Erasing & Getting the photo ready to place on Mario
Use the magic wand tool again to erase the majority of the background of your photo. Now use the eraser tool to clean up the background pieces that the magic wand missed. You can adjust your brush size to work better. Now you have just the body of the person. In this case I only need Noack's neck & head so I can erase the rest of his useless (for now) body parts, starting with his arms, then his legs, and finally his torso that he is merely a floating head. I place a box around his head to select it then I go to edit then copy. Then I switch to the other photo tab and click edit, paste to add Noack's head into the Yoshi photo.
Step 3: Placing Noack's head on Mario
Click on edit, free transform to size the head to the proper size to fit over Mario's. Click the squares around the rectangle and drag them out to your desired size. Once you are done hit enter to apply the transformation. Click onto the background layer and start erasing the parts of Mario that peak out from behind Noack's head, including the hat. Now click onto Noack's head layer and erase the extra parts of his head which are unnecessary.