Step 1 Open the PSD file, every mockup has more or less the same layout. Look in the layer panel (in the right bottom corner) for the a smart object labeled "Your design here" or "Add content here" or "Replace this layer" or something like that. Double click that layer. (see image below)
Step 2 A new window should open. Paste or import your design in here and sure it fits the canvas. (You can always transform your layer by using CMD+T or CTR+T, then scale your design to fit the canvas.) (see image below)
Step 3 Just close this window. A pop-up pops up asking to save the layer. Press save.
Step 4 Tadaaaa! The mockup featuring your design is now ready to be used everywhere you want!