I basically followed Sea Lemon's YouTube videos, following the links from this one.
To start, I opened all the envelopes and removed all their contents. Then I sorted envelopes and papers by size. I set aside magazines, postcards, and double-sided pages.
To work with envelopes, I unglued and cut off the flaps before cutting the envelopes to size and folding them to make text blocks. Although I tried to make each book with only one size of paper, it wasn't always possible. To accommodate, you could cut the papers to the same size. I just ignored the different sizes. These journals will be for me to write in, anyway. The number of signatures varied from book to book (anywhere from six to fourteen so far). For the endpapers, I ripped sheets from magazines or other double-sided pages and cut them to size. I only used book headbands for my first one because the spine wasn't even and I wanted to hide that. I did not add a bookmark.
For the cover, I used postcards or the covers of booklets for the inside. For the bookcloth portion, I cut a piece of colored paper to size. Then I glued on logos, stamps, and addresses from the cut-off envelope flaps. Finally, I used a glue stick to glue a piece of white tissue paper over the whole thing. I treated the entire thing as the bookcloth. I like the effect of the tissue paper because it "washes out" the colors. Note that the tissue paper is very fragile and will likely tear unless every inch of it is glued down. I also ran out of tissue paper; I have been using a thick, clear, flexible plastic that some of the mail came in. It is harder to glue, but it gives the books a nice sheen. I also used a Chipotle bag for the covers of some books. I like papers that have text on them. You could also decorate with stickers or drawings.
I really enjoy making these books (twenty and still going). It's a great way to recycle paper, and because I'm a daily journal-writer, it's also very practical. (I just find white spaces to write.)
Now I just need to figure something out for the magazines, postcards, and double-sided sheets!