It's called "loading" the tires. As stated, 3/4 full is the norm. Both rear tires should be filled. Filling the front tires is not as common. You need to set up a pump/vent rig to get this done and there are lots of YouTube videos that can show you how to do it.
If you are in an area that sees freezing temps, you need to use a liquid with a low freezing temp. Ideally, you want to use a heavy liquid so you get more ballast weight per gallon filled. Common options include:
- CaCl used to be the standard, and it's 1.4X the weight of water, but can (will?) cause major corrosion issues if you leak even a small amount.
- Windshield washer fluid is readily available, and relatively inexpensive. But it weighs less than water (~.8X) so you you get less bang for your buck.
- Mixing anti-freeze is another readily available option, but if you leak even a small amount, you risk poisoning your animals, as mammals are very attracted to it and it is deadly poisonous.
- Beat Juice is widely considered the best option as it is 1.6X heavier than water, doesn't freeze, and isn't toxic (though I hear it will stain your white shirts). The problem is that it's not available at Walmart. You have to go to a tire store that services commercial vehicles (trucks and tractors) to get it. There are a couple brand names out there... Rim Guard is one. It's very viscous (similar to molasses), and you'll probably have to have it filled by the same dealer.
I only know all this because I had to put new tires and rims on my Ford-New Holland 1720 4x4 tractor this past summer. After 28 years in service, the ag (bar) tires were only about 1/2 worn, but they had quite a bit of dry rot. They were loaded with CaCl when I purchased the tractor (second hand, but only a couple years old). Over the years, I had leaked CaCl several times while adding air to the tires. It turns out that this stuff is
sticky and doesn't wash off in the rain. It also seeps into the rim gaps and cracks and rusted it right through. New rims, new tires, tubes and beat juice fill set me back nearly $1K. I had to set up a pump rig to drain the tires in order to handle them, as they weighed ~350 lbs each. Now I have ~50 gal. of CaCl stored in kitty litter jugs that I can't get rid of.