I put our promo cards at coffee shops and laundromats, emailed meetup groups, went door to door, bought people coffee to demo our product at Starbucks, walked around farmer's markets and gave people coupons and samples, and literally pounded the pavement.
Then, you follow up with them personally. Do they want to order again? Why or why not? What would make them order again? Are they willing to let you visit them in person? What patterns are you noticing in the early feedback?
High level: first you develop a small core user base, then you learn everything about them and communicate with them constantly to find what is secretly working and not working. I would go to people's houses and talk to them and watch them eat dinner multiple times per week. That led me to our 10-minute dinner kits. The AirBNB founders went door-to-door to people's houses and helped them take the right photos to market their homes. This grit is the difference between the want-rapreneurs and the entrepreneurs.