For those of you who can't make it to the Chicago Creative Reuse Exchange or the Waste Shed for in-person workshops, here's my li'l wiki how for you. There are loads of techniques you can try and there is probably not a wrong way to do this (except maybe balling up the paper, which would result in less of a journal and more of a ball).
Step 1: Collect junk mail (yours, or another person's, will get the job done). For this example, I'm using a tri-fold flyer that is sort of thick, but not cardboard. (It is also shiny, which I personally dislike, but we can cover that up!)
Step 2: Fold the center of the tri-fold accordion style. You can just do one fold. Here I managed two, but you can usually get three or more folds with thinner paper.
Step 3: Use scissors (or brute force, I guess) to trim the top and bottom off of your accordion fold. I used IKEA scissors, and I want to put a little note here that the cheapest scissors you can find _will_ cut crooked. It's okay if that's what you're going for, but it's not so great for miniature or precision collage work.
Step 4: Take two pieces of paper (regular notebook paper, pages from a magazine, construction paper, or file folders work if you want something sturdier) and fold them in half. (I used two pieces of paper because I have two folds - use as many as you need for the number of folds you made.) I used graph paper with hole punches because I knew I would be trimming the hole punches off (although you could always leave those as a creative feature).
Step 5: Trim your paper to fit your outer cover. (You can leave the pages larger, or make them much smaller, depending on the style you are going for. I generally find it helpful to use the cover as a guide because that's my preference.)
Step 6: After trimming your paper for the inside pages to the size you like, place the folded edge along your accordion and mark the top and bottom of the accordion fold on your inside page (in this case, the red arrows on my graph paper below).
Step 7: Make an opening between your marks. You can cut this line with an exacto-knife, if you like precise cuts, but cutting the opening with scissors works fine for my aesthetic (a little more handmade / loosey-goosey).
Step 8: Fit your inside pages onto the accordion folds. (This is why it can be useful to have a sturdier paper as the cover and binding.) Widen the opening as necessary if your page doesn't fit (or trim the page as necessary if it winds up being larger than you want). Repeat this process for all inside pages.
Step 9: Find some more paper. I used a heavier postcard for this part, but you could also use ribbon, paper, cardboard, a Popsicle stick - anything that you want to use as the "lock" for your inside binding. (The papers you use can be patterned or blank - you can cover up most anything you want to cover up on patterned paper, or you can leave a blank book open to creative possibilities!)
Cut whatever you are using so that it is slightly less than the width of your accordion fold. Make sure it is at least slightly longer than the accordion fold. You can measure with a ruler or just go by how it looks like it lines up (vibes-based measuring - I find this to be a very forgiving project in terms of precision, but, as in all things, your mileage may vary).
Step 10: Insert your locking paper into the accordion fold. (The length of the locking paper will hold your inner pages in place.) You can use a glue stick to secure the locking paper, or fold one end, or use staples, or try washi, or apply rainbow duct tape, or just leave it loose, or cover it with additional pages - lots of options, all of them fun!
Step 11: Repeat the process for any additional inner pages.
Step 12: Enjoy! You now have a junk journal to decorate as you wish!
Step 13: Be extra and keep making junk journals with varieties of sizes, shapes, and topics. Here's an example of a tri-fold with a different accordion binding style and inner pages cut from old file folders: