How do I find all the freeze plugs?

Auto parts stores have books that show the locations, and they have complete kits to include every plug of the correct size. There should be three on each side of the block, and there's going to be one on the back, but you have to remove the transmission to get to that one. The kits usually include smaller threaded plugs too. You can also buy the plugs separately.

You don't necessarily have to replace all of the plugs at the same time. It's fairly common for one or two to corrode through and start leaking while the others look like new. I've found the corroded ones usually have a handful of casting sand stuck behind them that didn't get shaken out when the block was cast. That sand gets removed during the engine's first cooling system flush. My suspicion is that sand prevents the additives in the antifreeze from circulating past those plugs. The main additives are water pump lubricant and corrosion preventers. Those additives wear out in about two years which is why we're always told to replace the coolant that often.

There's a special tool made for installing these plugs to the correct depth. It's just a long handle and various sizes of cones that can be attached and swiveled as necessary. You use a small hammer to drive the plugs in. Most auto parts stores rent or borrow tools so you don't have to buy it and never use it again. In my city they make you buy the tool, then you get a full refund when you return it. If you choose to keep a tool, you still return it, then they order or give you a brand new one.