While it can be a great adventure, it can also be very dangerous. So my recommendation would be that you find a knowledgeable mentor/instructor in the area and learn as much as you can. Book learning is great, but no substitute for experience. Go to some local horse shows and ask around to get recommendations on instructors in your area who may be able to assist.
I would also recommend that you consider using fly predators from Spalding Labs for your insect control. These are insects that you release monthly during the summer months. They bore into the manure and eat the fly larvae so that flies do not hatch. They work better than any fly spray and they work 24/7 which no fly spray can do. Cleaning the manure daily and removing it to a remote location is also a great way to reduce the bug problem as well as reduce the chances of your horses getting worms. Ask your vet what to feed them based on their present condition and availability within your local area. There are lots of different ways to feed, and different horses have different needs. IMHO it is irresponsible of anyone who has not seen and/or worked with your horses to recommend a feeding program. Generally speaking horses are foragers and more readily gain weight by eating hay rather than grain. But there are lots of different kinds of hay and some horses do better on one kind than another. If your horses are thoroughbreds, chances are they may have ulcers which can be improved by feeding a diet with some alfalfa. Most horses do not need that high of protein unless then are in serious work. Slow feed hay nets are great. Feeding grain in smaller amounts multiple times each day is better than once a day. Horses digestive systems are designed to process food 24/7 and their stomach acid continues to produce even if they do not have anything in their stomachs to digest which causes ulcers. So feeding small meals often and/or using slow feed hay nets to distribute their hay throughout a longer period is the best way to minimize the risk of ulcers. Abundant, fresh, clean water is essential at all times. Shelter is also very important. There are lots of opinions on blanketing, but Mother Nature designed horses to exist in the wild without blankets so in most circumstances blankets are not necessary, particularly in California. In weather 50 degrees or colder and wet, they may need some assistance to keep warm if shelter is not available. Have your vet do a fecal count to see if they have worms and only worm if necessary. Worms are developing resistance to the wormers so they are becoming less effective. So much to know....good luck!