Just look up an uncle Ben’s tek method of mushroom growing. It’s a technique for growing illicit mushrooms but works the same for all mushrooms as far as I’m aware.
Also you’ll need to look up a tutorial on how to transfer your spores from the spore print into a syringe so you can inoculate the bags.
I would encourage you not to grow Agaricus Bisporus in your garden outdoors if it's not native to wherever you happen to live. Mushrooms can't really be contained as easily as plants so it could become invasive very easily. Or it will just die if your climate is too far off from what it's used to.
If you're looking for an easy way to get started with mushroom cultivation, you might want to try one of those oyster mushroom kits you can find online. Just follow the directions and grow the initial crop of oysters, then if you're so inclined, you can use the block of mycelium from the kit to start again. To reuse the kit you need a plastic turkey bag, alfalfa pellets (rabbit/guinea pig food from a pet store), and Timothy hay or newspaper pellets (both used as rabbit/guinea pig bedding so same aisle as the alfalfa).
Sterilize the turkey bag and boil the mix of alfalfa/hay/paper (the substrate). When the substrate cools layer chunks of mycelium from the kit with layers of wet substrate and pack it down tight so there's no air in between.
Leave the bag in a warm dark place until the mycelium covers the substrate (a couple weeks or so, check about once per week). Then bring it out and cut holes in the bag and put it in a tub or box. From there it should be the same procedure as the kit, mist it a couple times per day and fan it to get some fresh oxygen in there.
You can keep repeating this process to get more oysters. If you want an oyster log, you can stick piece of a dowel in your substrate (sterilized of course) and wait for the the mycelium to grow on to it. Drill a hole in your log and stuff the dowel in, with any luck the oysters will take over the log and you'll keep getting oysters until they eat the whole log.
That’s quite detailed, I’ll save it thanks!
Based on a quick search, they grow worldwide & didn’t show up when I checked for it as an invasive species. So, it should probably be fine to grow in California.
I'll second the suggestion to start with oyster mushrooms (which you may still find at some grocery stores) rather than agaricus, but for different reasons.
Oyster mushrooms are really easy to grow. You can buy kits, or you can do it from scratch in a 5 gallon bucket or garbage bag with straw or wood shavings or similar substrates.
Agaricus, on the other hand, are more complicated. They're not primary decomposers like oysters, they're secondary decomposers. Which means rather than using a fresh substrate, you have to grow them on active compost. It's certainly achievable by a dedicated hobbyist, but it's not going to be an easy first time IMO.
If you want a good chance if success, start with oysters.
Edit: oops, just realized this post was months old.