The two most common ways to cut hay are with a disc hay mower and the tried-and-true sickle hay mower. Disc mowers cut faster and don't care much about moisture, but sickle mowers give a cleaner cut and allow the hay to recover and grow back faster, which is a major plus for multiple harvests in a single growing season. Sickle mowers are also easier to repair and take less power to use, but the trade-off is that it needs to be run at a slower speed for good cutting results.
Cutting hay with a sickle bar mower requires one very important requirement: the hay has to be dry from atmospheric moisture like rain or heavy dew. Waiting until mid-morning or early afternoon is the best time to begin cutting. The sickle mower should be either tractor mounted or self propelled for smaller hayfields. The tractor should have hydraulics to extend and cut with a sickle bar mower.
Walk behind a self propelled mower and lower the cutters to near ground level and do not allow the blades to come into contact with the ground. Hitting the ground will not only dull the blades, resulting in an inferior cut, but they can also break, especially if they hit rocks. Although broken blades can be fairly easily replaced, the more time you spend replacing blades means less time cutting hay. This same rule applies to tractor lowered sickle mower bars.
Allow the blades to do the cutting and do not rush the process. The sickle motion cuts like a scissors, and the faster you go, the sloppier the cut will be to the point of missing whole swaths of hay. Take your time and let the blades do the work.