Sorry. Gotta disagree. Rivers always flow downhill, of course, but they seldom do it in a straight manner. We'll disregard those rivers on seacoasts that sometimes flow upstream and sometimes flow downstream near the coast, based upon the tide.
Sorry. Gotta disagree again. There are many rivers that never reach the sea. In the U.S.A. you could think about the three rivers that flow into the Great Salt Lake and end there. Nothing flows out of the Great Salt Lake. Or you could think of the Truckee River in the Sierra Nevada Mountains in Northern California. It flows out of Lake Tahoe down the mountains to Lake Pyramid in Nevada, and the flow ends there. Nothing flows from Lake Pyramid to the sea. There are many more similar examples.So start your rivers at the highest ground (mountains usually) and wind them around hills until they reach the sea.