DON'T!!!!
Buy it.
I worked on POS software for years. I'm glad to be away from it because it's *HARD*. A lot harder than you would think it would be.
For a donut shop, even a small one, speed of service is important. So is reliability. (You really, really don't want to hold up your customers because your home brew program crashed AGAIN and you have to reboot). Designing a touch screen order taker that's easy to use and easy to learn (you're going to be hiring people from the low end of the spectrum, right?) isn't as easy as it sounds.
Do you plan on taking credit cards (if not now, in the future?) That's not a trivial task if you want it integrated with the POS rather than a standalone thing.
If your business is successful, you'll find you want more and more things out of your POS. You may not think you need all the features the commercial packages offer, but as time goes by, you may find you really do need them, and trying to do that yourself at the same time you are trying to grow your business will just be a major headache.
In the 10+ years I worked in POS, I saw a number of customers who thought they could 'go it alone' and write their own POS. These were bigger (fortune 500) companies. They all failed.