As with any business, you would need to sell your product before you make it. If your product is unique enough and valuable enough then clients will pay you upfront. You’ll also need a good salesperson who works on commission. With enough purchase orders in hand, you can find the funding to create the product.
For example, you might sell enough popcorn that you could buy the materials and have a contractor make it for your orders, After enough time and profits, you could buy the popcorn making machinery yourself. But this way you could start from nothing.