As I reklect, there was a pattern or two in the old Mountain Man's Sketch Book years ago...For myself, I never really used a pattern but just decided what size I needed and sorter improvised.
Granted, some of my possibles bags were no things of beauty or anything...but they did work. What I typically do when making something like a possibles bag is to visualize what I want, draw it out on paper, and then make a prototype out of cloth to see if if's what I want. Obviously you've got to bear in mind that leather is going to go together much differently than the material from an old sheet.
In fact I've often thought that a thick towel would be a better pattern maker for this...though I confess I've never tried it.
As for materials, my tastes run toward a fairly heavy leather but one that is soft and supple. Many shooters prefer the stiffer leather such as you would find on an expensive leather satchel.
For stitching material, I prefer artificial sinew. Tough as can be and pretty easy to work with. (Now I must confess, I haven't made a possibles bag in years and wouldn't know where to purchase the artificial sinew today. Our local buckskinner supply store went out of business years ago.) A good quality awl surely helps with stitching with this material but I have also used large eyed needles. I think I bought both at Tandys.
I have also used leather to lace up at least one possibles bag. A leather punch sure helped make that operation easier for me.
I realize that this info isn't much in the way of a 1-2-3 explanation of how to make a possibles bag but it's the way I've always done it.
Good Luck and have fun! Makin' your own stuff is a big part of the fun of black powder shootin' and carryin' on!
Rocky Point Jack