Ensure that your file has been downloaded and unzipped to use. If you are unsure how to do this check the tutorial here.
1- Open Affinity Designer. Select file from the top panel.
2- Select the Mock up file you would like to use. You can open Photoshop files in Affinity Designer.
3- Go to the layers tab on the right and select the logo layer.
4- The logo will highlight. then on the left tool bar select place image icon.
5- select the file you would like to use on your mockup. the file needs to ideally be a PNG file with a transparent background. this way it will look like it is supposed to be there on your bag. (if using an image to put inside say a iPhone mock up the image does not need to be PNG then)
6- The image will appear onto the mock up over the original mock up image.
7- Go to the layers tab and select the original mock up logo. Untick the box so the logo is hidden and only your image is on the bag. Adjust your image to where you would like it.