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The graphics and slogans used on your shirts don't need to be complex, but they must be original enough to stand out in a flooded market. You can design everything yourself if that's your area of expertise, but if it isn't, work with a designer to create something that works. Check out online design communities like Dribbble and Behance, or freelance networks like Freelancer and Elance. Search through the work of existing freelance designers in these networks and contact the designers you like about working with you. Before finalizing your design, check the way it looks on digital mockup images. Since customers will want to see the design before buying, you'll need to make these same images available to customers once you start selling.  You could use photo imaging software and digital T-shirt templates to create your own design previews. You can even use design tools offered by online companies to create your T-shirt design. Alternatively, you can skip the digital mockup and simply print one sample shirt with that design. If you choose to sell that particular design, take a digital photograph to show your customers how it looks. Take time to choose the right type of blank T-shirts for your business. Aside from the cost, you also need to choose blank shirts made with comfortable material. The fit and sizing of those shirts should be agreeable, as well.  Do your own research and make a choice based on your own interests and needs. A few popular T-shirt blanks include All style , Alternative Apparel, American Apparel, Anvil, Gildan, and Tultex.  Once you narrow down your choices, order a blank from each company and check the product itself before finalizing your decision. No matter which print method you choose, you'll need to hire the services of a professional printer. Do your research to determine which printers might offer the best price and service for your business's needs.  If you print using heat transfers, you should look for local print shops that can work with you. You'll also need to invest in a high-quality heat press machine  If you use screen printing or direct-to-garment printing, find online printers with good reputations. Note that screen printers will often print batches and ship them to you, but direct-to-garment printers will usually print individual shirts and ship them directly to the consumer. Order or print up a sample version of your product to check the quality of your merchandise. You'll need to sell high-quality products if you want your business to survive past the first few months. If you're dissatisfied with the way the product looks, figure out what the problem is and correct it. The design might not look as good in print as it did digitally, and the problem may lie with the design itself, the printing method you chose for it, or the company doing the printing. Run the design and final product past potential customers before rolling it out and trying to sell it. Make sure that there's interest in a design before you invest the time and money required to sell it.  Post images of the design on social network pages. Understand that friends and relatives might be biased toward you, however, so some of the feedback might be idealistically positive instead of realistically so. Consider running a crowd-funding campaign. If people like your design well enough to pledge money to it, the design itself has good odds of success.

Summary:
Create unique designs. Check your design. Use high-quality materials. Find the right printer. Test the quality of the final product. Get feedback before opening shop.