1. Brew 2 shots of espresso, that we know for sure
2. Prepare about 4 ounces of milk, frothed with very little aeration
3. Pour milk in your espresso
That is quite a tricky one. Just because there is no common understanding of what it really is. Originally it was quite like a cappuccino but without sea-foam picks, i. e. flat. So, not a cappuccino anymore. Through the years it transformed into something close to latte but with less foam.
As for now, general consensus is that a flat white is smaller than a latte and contains more espresso. Differs from cappuccino by being stronger and having foam with more velvety texture and glossy finish (while cappuccinos have fluffy and airy foam).
Remember that velvety and smooth foam seems to be the main difference as it also affects how milk melts into coffee.