There are plenty of guides out there on how to setup the Shopify store, pick a theme and all that. The entire part about adding a service like Printful etc can be challenging for a non-technical person, but it's not the most difficult part.

The toughest part is knowing how to craft a proper marketing strategy.

The basics are:
- Know your ideal customer. What they love, what they stand for, where they hang out, what other things they like
- What they're already buying
- Crafting an irresistable offer and upsells
- Creating marketing campaigns, a following on social etc.

I'd say the most important and critical part to any print on demand business is to rally a tribe around a central message / cause they'll all raise a glass to and waive a flag for. Anyone can sell cool looking t-shirt designs... but to really create a loyal audience, it has to have a central focus.

That makes the design picking easier, the FB/Google ads easier, the email writing easier, the product upsells - all because everything is directed to one group/audience.