How to sew a romper

Morning! I'm back again picking your brains (god I hate that phrase) over Payhip. I've been reading up on Dave Smyth's website but I'm still confused over the simpliest thing - if I have a shopify site, do I install Payhip as an app? Or do I redirect people to Payhip at POS? Or do people have to find my product on Payhip as a platform?! TIA xx