How Do You Ground Flaxseed

I give ground flax to both my horses.I buy it whole a 50lb bag from the feed mill and ground it for a weeks worth or so.Its fine for winter, but in summer you have to be careful about it going bad.Buying it preground is usually alot more expensive, especially if you buy small quantities at a time.I think they get about 12 to 23 scoop a day.