How to use these mockups?

I've just downloaded a 7-day trial of Photoshop CC so I can experiment with these mockups: https://mockup.zone/print-flyer-poster

I've been using Gimp/Inkscape and not been involved with print much until now. So my question is; the mockups have folded pages etc. How do I make my image 'transform' properly - is that feature built in to the mockups? I've tried skeywing my image a little and making it semi-transparent to let the paper folds show through, but it weakens the image too much and generally looks quite bad

I know there's a trick to getting my mockups to look as faithful as the demo at the link above. They're just .psd files. Can anyone point me in the right direction?

Thanks!