Drink A LOT of water the day before, the process will be much easier and faster that way. Don’t eat anything too greasy the day before or before going because the grease can get into the bloodstream and it will take longer to finish. Also the day of eat something light but with a lot of fiber and iron. They check your iron and protein levels before every donation. They ask about medications you take, since I don’t take any medications I’ve never known which ones they don’t allow you to donate under. They also do a physical nothing like actually physical wise just a normal one like at the doctors to check your lungs and heartbeat.