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
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.
Step 2: The bag
An added bonus beyond ease of installation is that it will also be easy to remove the armor for any reason from, it's heavy to I'm going to the airport and have no idea if TSA prohibits this sort of thing nor do I wish to find out even over the phone.
Step 3: Measuring the armor and cutting
I didn't make a pattern, just laid out the pieces and marked where to cut with chalk. Not at all tricky.
You might use large, sharp sheers if you have them. Whatever is handy and very sharp. Probably not going to cut it in one go. I have no idea what armorers use to cut this stuff. Probably big sheers or some industrial cutting machine.
I used a rotary cutter and sheers.
Step 4: Make a simple sleeve for the armor plate
This is very straight forward. Just follow the photos and the notes.
Step 5: Securing the plates
Step 6: Value Added Extras
I did add velcro for patches on the strap facing up and slightly forward. Right now it holds and IR reactive US flag patch that lights up pretty bright when viewed with night vision. Probably not necessary but it makes you visible to authorities that would be the most likely people using night vision and identifies you to at least some extent as someone who isn't hiding from them. If you already know any more (or less), that explanation of these patches should be sufficient either way.
As an after thought, other than the fact that I have blasted my project across the internet, there really is no way anyone would suspect the plate in it's t-shirt casing is anything other than padding. If someone is dismantling your bag deeply enough to figure that out, you've got worse things to worry about anyway.
Step 7: Important information for everyone!
A very special thanks to Don at Box O' Truth for putting this out there for the benefit of all of us.