Okay, black powder is always measured by volume instead of weight. You could use both of those powder measures but you will need something to measure your shot with also. Most shot pouches can be adjusted according and initially you should make the powder charge equal to the shot charge using the same volume. In other words and for example 2 ½ drams of powder and then add 2 ½ drams of shot to keep it equal by volume NOT weight. Your gun is older I would take it easy on the charge you put in it.
The thinnest card goes over the shot, don’t worry it will hold. The thicker card can either be used in conjunction with the cushioning (fiber) wad or alone by itself directly over the powder. In my loads I dispense with that hard thicker card and only use the fiber cushioning wad over the powder, then the shot and then an overshot wad, again, that real thin one. My shotgun just patterns better with that combination but you will have to experiment also to see how your shotgun patterns.
Be careful, keep your face and as much of your hands away from the business end of those barrels as much as possible. Half cock the hammers to allow some of the air to escape while you are pushing those wads down those barrels. The wadding is usually very easy to push down a barrel if the barrels aren’t badly pitted.
Loading a rifle is dangerous enough but a double barreled shotgun is in a league all by itself and I swear sometimes you can see the devil himself staring back up at you while you are loading those barrels.
