Howdy
Although I always use FFg myself there is no problem using FFFg in shotshells in your Stevens. We are not talking muzzle loaders here. It will not raise pressure all that much.
With all due respect to Mike's video, there are a couple of other ways to load BP in a shotgun shell. A lot of us do it for CAS.
I recommend you get yourself either a Lee Load All
http://leeprecision.com/shot-shell-reloading/shot-shell-reloading-press/
Or a MEC Jr.
http://www.mecreloaders.com/ProductLine/600JrMark5.asp
There are plenty of both of these on the used market.
If you use one of these machines you do not need to be cutting hulls, the press forms the crimp for you. And you usually will not need an over shot card because the crimp should hold everything in place.
There are two basic ways to load Black Powder into modern shotgun shells. Either you use modern plastic wads or you use separate card and fiber wads. By far the easiest thing to do is to use modern plastic wads. That is what most guys do who shoot Black Powder in their shotguns in CAS. The most popular wad most guys use is the old Winchester Red Wad. The problem with most modern shotgun wads is they are too tall overall and do not leave enough space for the powder charge. That is fine for a Smokeless charge but you need more volume for a BP powder charge. The old Winchester Red Wad filled the bill, but unfortunately Winchester stopped producing it a few years ago. But fear not, Claybuster makes their own version of the same wad.
http://www.claybusterwads.com/index.php/product-by-style/by-style-winchester
CB1138-12 (WAA12R), the 7th one down.
Using plastic wads is much simpler than using separate wads. The drawback is that since Black Powder burns hotter than Smokeless, a little bit of the wad melts on its way down the bore and there is usually a bit of melted plastic left in the bore. This is not a big deal, lots and lots of guys do it. You just have to get the plastic out again when you clean the shotgun. Usually allowing the barrels to sit for a while filled with hot water will loosen the plastic so it can all be driven out by a damp paper towel shoved down the bore.
The other option, which I prefer, is to use separate wads. Yes, you can make your own, but I prefer to buy mine. I buy them from Circle Fly.
http://www.circlefly.com/
Track of the Wolf also carries the Circle Fly line.
You are going to need an Over Powder (Nitro) Card and a fiber Cushion Wad. You may or may not also need an Over Shot Card, depending on the total height of your powder/wad/shot column.
For 12 gauge I use the 1/8" thick Over Powder Card, and the 1/2" thick Fiber Cushion Wad. The Over Powder Card seals the powder gasses behind behind the shot and the Cushion Wad does just what it says, it cushions the impact of the expanding gasses to the shot.
A Square Load simply means the same volume of shot and powder. Do not get all hung up in the business of measuring powder by weight or volume, it is not worth the argument. Very generally speaking, if you put in more powder than shot, by volume, you will tend to over power the shot and blow a hole in the pattern. I hear guys in CAS bragging about how much powder they stuff in the hull, and then they wonder why they cannot hit anything. Equal volume of powder and shot works fine. However if you want a little bit less recoil, putting in
less powder than shot, by volume, also works, and that is in fact what I do.
If you use the MEC Jr, it comes with a standard powder/shot bar that delivers 1 1/8 ounces of shot. That is what I use.
Get yourself a set of Lee dippers. I do not want to get into any arguments here about static electricity and Black Powder and plastic. I simply never put Black Powder into the plastic powder bottles of a shot shell press. Instead I dip my powder. I use the largest dipper in the Lee set, the 4.3CC dipper. Not all Black Powder weighs the same. That is part of the problem with the weight vs volume argument. In actual weight, when I use the 4.3CC dipper, I am using about 64 grains of Schuetzen FFg or about 58.5 grains of Goex FFg. Notice I said
about! If you weigh it your self you may get different weights because BP can also vary by weight from lot to lot.
So, here is my 12 gauge BP recipe.
12 gauge hull, I prefer the Remington STS.
Any shotgun primer, I usually use Winchester.
4.3CC of whatever brand FFg. (or FFFg if that is all you have)
Circle Fly 1/8" thick Over Powder Card
Circle Fly 1/2" fiber Cushion Wad
1 1/8 ounces of shot (I like #8 myself but you can use pretty much what ever you want)
Circle Fly over shot card.
This load is actually less than a square load. The powder volume is less than the shot volume. As I said before, it is no problem and will not over power the shot. The reason I use the Over Shot Card is because this powder/wad/shot column does not quite fill up the hull and the crimp is slightly concave. This sometimes leaves a small hole at the center of the crimp and some shot pellets can leak out. By placing the Over Shot Card on top of the shot it seals the shot in and none can leak out.
I use my old MEC Jr. First I decap and resize in station one as usual. Seat a primer in station two as usual. Then I pull the hull off the press and dip out my powder, pouring it in using a small powder funnel. Then back on the press at station three I seat my Over Powder Card using the ram of the press. This does not push the card quite all the way down, so I take the hull off once more and use a piece of 5/8" wooden dowel rod to seat the card against the powder. I lean on it a bit and listen for a slight crunch. That is plenty of compression.** Then back on station three on the press and seat the fiber cushion wad, then drop the shot, then seat the over shot card. Over to station four to start the crimp and station five to finish the crimp.
Taking the hull off the press twice does slow things down a bit, but I can still crank out about three boxes an hour this way.
If I were to be using plastic wads I would do everything the same except I would only seat the plastic wad at station three before crimping. I would have to see how the crimp looked to decide if I needed an over shot card or not.
That's it, pretty simple, been doing it a bunch of years this way.
P.S. Most older shotguns had a relatively short forcing cone. separate wads worked fine with these. Most newer shotguns have a longer forcing cone. Modern plastic wads bridge the longer forcing cone a bit better. With a modern longer forcing cone separate wads can tend to upset a bit in the forcing cone, and this can affect your pattern.
But even with separate wads, my old Stevens hammer double which has had its forcing cone lengthened, always knocks down anything I point it at.
**Safety Note. When compressing the powder with the dowel, I make sure to keep my head and chest clear of the dowel. Just in case. I have my MEC Jr mounted on a piece of wood, and I drilled a flat bottomed hole in the wooden base. I seat the hull in this hole when charging with powder and when compressing the powder. Just for safety's sake I always make sure a stray piece of shot that might contact the primer has not found its way into the hole when I am compressing the powder.
So it will be the candidate for the BP loads. I watch Mike's videos on loading without expensive equipment and I understand the basics but have a couple of detail questions. I've read about the "square load" where you powder volume matches your shot volume (80grs of powder=80grs of shot). 
