There are thousands of designers in the world today creating KILLER designs and weirdly enough - most decide to use pre-made mock ups that others have made. Our main question is this - WHY? If you're talented enough to create a design worthy of getting printed hundreds of times and worn around the world, why would you use somebody else's platform to showcase your hard work? The simple answer is this - creating a professional mock up seems daunting, time consuming, tedious, and requires professional photography with proper lighting, etc.
Heres a hint - THATS NOT TRUE. I'm going to give you the rundown on how to create a mock up from scratch that is unique to you and your brand and can create originality in what you're trying to portray with your design style. You're better off trying to borrow a friends DSLR if you have one available - but for this reference i shot the image with my iPhone 6 to show how simple this process really is.
Start by taking any blank shirt you have laying around and run it through the wash. Once its dry, hit it with a lint roller to avoid any annoying post work you might have later in PS. For this, I cleared out a spot in my living room and laid the shirt down about 5-10 feet away from the window. Try to avoid harsh lighting as this will throw off the look of the final image.
Using the magnetic lasso tool (or any cutting tool of your choice) go around the edges and cut the shirt away from the background. This is going to help you in the next few steps when creating the shape layer for deciding what color you want to make your mock up.
Since i took the image at a slight angle, I went ahead and squared it up to give is the true head-on feel for the mock. I also desaturated the cut out so were only working with black-white pixels. I also dodged the upper-right corner of the shirt where i lost some detail in the shadow of the image. This balances out the light so I can get an even selection on the next step.
Navigate to the menu and click Select > Color Range and click on one of the darkest areas of the shirt, This step is really going to require your eye to gauge what your selection looks like. The goal is to get an even selection of shadows across the shirt to place over top of your shape layer that you'll be using to color the mock. Create a new layer and fill this selection with black. This is going to be the layer that goes over the artwork and will sell the effect of a true printed shirt. Rename this layer to "shadows"
Hide the shadow layer and select the cutout below it. Duplicate the layer and select the color you want your mock up to be. If you'd like the mock to be black - aim for a darker grey color as the shadow layer will prove the full black shadows to finish the look.
Activate your shadow layer and viola! You have a mock up! Now all you need to do is add your artwork. navigate to the image you'd like to display and drag it into your mock up PSD. Put your artwork layer underneath your shadow layer and adjust your blending options to either screen or multiply if necessary.
Once you're done - you can change your shirt color using the blank shape layer underneath the artwork. Double click the layer and select color overlay and color until you find what you need.