To do this we are going to use the Transform Tool to gently curve the artwork so it looks like it would if it were on a mug. Observe how much the curve of the mug appears in the photo and as a general rule curve a little less. This should be a subtle change on a mug that is shot straight on like in the example. Your can buy the mockup photo in Etsy at https://www.etsy.com/uk/WanderlustLens/listing/710515692/photo-for-mug-mockup-woman-holding-mug?utm_source=Copy&utm_medium=ListingManager&utm_campaign=Share&utm_term=so.lmsm&share_time=1564265536807
Open the JPG file in Photoshop. Make sure the Layers panel is open
Under File >Place Embedded. This will open your files. Chose the file with your artwork.
Resize by grabbing the corners. Press Enter or select the tick
Press Control-T (Win) Command-T (mac) to transform
Right click within the artwork bounding box on screen and choose warp.
Grab the internal linesof the grid and/or the dots to slightly curve the art so that it reflects the same curve of the top and bottom of the mug.
When happy with the results, click Enter or select the tick.
Please see the video below for a demonstation of these photoshop techniques.