Start by taking a paper bag (the lighter kind you get from a boutique, not the heavy kind you get from the grocer), and ripping/cutting it so it lays flat. Then trim it down to the size you will need to cover your book, leaving an inch or so of overhang an all sides for turning under. This technique is fairly forgiving, just make sure it is big enough. Then crumple up the paper bag a couple of times and flatten it back out again. Spread the matte medium over the entire surface of the paper (on one side) with a paintbrush. This is your glue. Go ahead and soak it down pretty good. Then apply the paper bag to the thing you want to cover, glue side facing the item. For my first attempt, I used a commercially available 5x8 blank book. To get the "veins" effect you see here, I scrunched up the paper a bit with my fingers, wrinkling it a little, as I applied it to the book. Then wrap the overhang around the edges of the boards, like a present. Tucking it in around the spine is the trickiest part. The rest is pretty easy. don't forget to put it in a book press while it dries (or cover it with wax paper and weigh it down with some encyclopedias) to prevent the water in the glue from warping the book boards.
At this point, it looks like a paper bag, glued to a book (which, it is). After it is dry, you can paint it any way you want. I experimented with a new technique that I just sort of thought of a few weeks ago (though real artists will likely be saying "Duh", again). I used a sponge to apply a base coat of black acrylic paint. Then, when it was dry, I applied a dark red in the same way, making sure to let some of the black show through. At this point it was looking a little less like a paper bag, but still wasn't very impressive. Finally, I dry brushed a brighter red and some silver across the whole thing. That's what really made it come together. Once the dry brushing was done, I was actually quite impressed with the results.
Before painting, I cut out a pentagram from a piece of thin cardboard and white-glued it to the front cover. I painted it along with the rest of the cover, but gave it a final dry brushing of gold paint to make it stand out. My last step was to install new end papers to hide the wrap around from the new cover. I used a black card-stock. I plan to make several more of these to sell in my shop. I also plan to incorporate this new technique into some of my upcoming projects. This was my first attempt at this, so I'm still experimenting, but I expect to make good use of it in the near future.