So, here's the steps:
1. Fold your poster board in half (like a hamburger for all of you elementary teachers out there). Try and keep the edges as even as possible. Elementary students usually need quite a bit of help with this, but middle school and high school students can pretty much do this on their own. Unless you are really OCD and have to have all of the edges just so, I would just let them loose on it.
2. Staple the two short edges. Close to the edge. I cannot emphasize this enough. You want your staples close to the edge.
3. Tape over the edges. I have used duct tape and masking tape. Masking tape is cheaper, but if you only have your students half a year it works just fine. If you have students all year long, you teach elementary kids who are pretty rough on their stuff, or you anticipate your students bringing their portfolios back and forth between home and school (which I do not recommend), use duct tape. Whatever you decide to use, make sure it has some width to it- 2" wide at a minimum.
After the portfolios are made, the students can "decorate" them. We always start with their names on one side. BIG. Make their names big, and bold, and creative, and whatever makes your heart happy. But easy to read.... With this group I decided to do their names in graffiti style. It seems to really catch their attention and of course it is really colorful and nice to look at! I used this tutorial to show the kids how to create graffiti style letters. And I also created this handout to give the students as a visual reference.
On the opposite side, we usually do some sort of elements and/or principles review.
I also highly recommend using a portfolio record. I have my students tape it down to their portfolios and fill it out as I pass back projects.