First caveat, this is not a cheap project. The bag is about $140 plus tax and the armor plates will run close $200 for new ones. You'll need access to a sewing machine and a few other minor things as well. All told it's close to a $400 project.
If you are a journalist, work late nights in retail or the restaurant industry or live in a bad neighborhood you might have very rational reasons to want something like this. If you are in law enforcement or military service you've already got what you need. That being said...
In all seriousness, there really is no reason most of us will ever actually "need" anything like this at all. This Instructable is for entertainment purposes only, as in, not even educational purposes. Building, owning or using body armor may even be illegal where you live. If you have been convicted of felony you are likely prohibited from owning or using ballistic armor. Committing even misdemeanor offenses while using body armor in some cases may elevate those infractions to much higher level crimes. Please to not build something like this and use it at any sort of protest or even while you on a graffiti run. If you get caught but weren't in real/serious trouble before, you will be when your bag is searched and found to be armored. This is in fact my giant disclaimer and warning. If you build this, you are on your own.
This Instructable is how I armored my bag to level IIIA+.
Step 1: The Armor
If you don't happen to have extra armor laying around you'll need to source some. I used standard run of the mill level IIIA armor with a small trauma plate. It's rated to stop "most" handgun rounds. The specs rate it to stop up to 9mm and 44 magnum. Though, it is important to note a few critical things.
A. That isn't a guarantee. Armor fails.
B. Unless you don't think getting kicked by a horse is a big deal, taking a hit from a 44 magnum even if your armor stops it will really get your attention. That's an awful lot of kinetic energy to absorb.
C. Armor wears out. Getting a cheap, beat up, surplus vest may not ultimately be a great idea. They wear out over time. If you are sourcing it and at all and serious about armoring something you may as well just buy new plates. They aren't terribly expensive compared to the value of not taking a bullet.
In most of the United States you can legally buy this type of armor everywhere from uniform shops to eBay. It's easy to find if you look. It is possible to get it inexpensively but again best to NOT buy budget body armor if you are going to buy it at all.