I've been setting up a few swaps on the bot and wanted to do one for mini file folders. A mini folder is my favorite way to package up a flat mail swap as it doesn't take too much paper and they look lovely when finished. Here are a few I have made using a variety of papers, come see!
I suppose there are a lot of videos on the tube but they tend to focus on specific measurements, and I'm not too good at that. That written, I'm going to post some pics of one way you can make a file folder using paper scraps. Some of the folders in the picture were made with 6X8 paper, one was made with a cut from a 12X12, and one was made with two pieces of paper to make it a bit larger,
Here are two pieces cut from a 6X8 piece of paper. I wanted to make the folder a bit wider so here I'm going to attach them together with washi tape both on the front, and the back.

Trim the excess paper and fold the left side over until you are satisfied with the size of your folder-to-be. Trim any excess off, but leave a little so that you can both see the contrasting paper, and cut a file flap. Ta-da! now you get to decorate.
A little washi tape down the side with a sticker completes the outside, and the inside utilizes one of the scraps as a belly band on the left where I have tucked in a few Halloween goodies, and on the right is a glassine bag with large Halloween glitter. Down below you will see a few other ideas depending on what you may have on hand.
This wee folder measures 3X5 and is decorated with a handmade watercolor potion bottle and some trim from the dollar store. Inside is a small assortment of Halloween cut aparts and stickers.
Limited on your Halloween supplies? This one was made from a sheet of orange paper, decorated on the outside with a handmade watercolor ghost, a few strips of black & white washi tape, and an old school dymo "BOO!" Inside there a vertical belly band on the right with a Halloween cut apart, a printed cat image from a freebie site, and a watercolor leaf. On the right is another cut apart, print out, a scrap piece of paper with corners punched, and a paperclip with a wee bit of lace knotted on it.
The key is to have fun and use what you have. Keep your items inside the folder as flat as you can muster, and under an ounce and your mailing cost should only be $1.04! Happy Halloween!