I like to re-use cardboard shipping envelopes for my simple looms. They're very sturdy, yet easy to cut with a pair of strong scissors. You can also use a sheet of cardboard, but I don't recommend corrugated for this project.
Draw a line 1/2 inch from the edge of the cardboard. This will serve as a cutting guide.
Use a pair of strong scissors to cut a series of slits in the edge of the envelope, 1/4 inch apart. Use that guideline you drew in the previous step to help you keep all these slits about the same depth. (But don't worry about being too precise, as you can see in this photo.)
Repeat these steps on the opposite edge of the cardboard.
Time to string your loom! I like to use worsted or sport-weight yarn. Leave it attached to the skein while you're stringing, and cut it off when you're done. Begin by wedging the end of the yarn into the first slit in the cardboard. Leave about a 3-inch tail hanging, as you see here.
Next, draw the yarn across the surface of the cardboard, and then wedge it into the first slit on the opposite side.
Here's a shot of the back of the loom. You can see that I've used two colors in stringing my loom. You can do this if you like, or not. (If you weave loosely, you'll be able to see the color change in your weaving. If you weave tightly, you won't.) If you do change colors while stringing, just leave a 3-inch tail of each color of yarn hanging at the back of the loom, like you did when you started stringing.
By the way, in weaving terms, these vertical, parallel strings are called warp yarn.
Ready to weave? Here we go. Cut about 48 inches of yarn. You can weave with sport, worsted, bulky, or novelty yarns—each one produces different effects. Thread one end on a tapestry needle. Pass the needle over and under the warp yarns, pulling the strand through as you go.
See what I mean? The second row should pass under the warp where the first row passed over it. And vice versa.
Pull the strand through to finish the second row of weaving. And then, keep repeating those steps to continue weaving. here are a couple fine points to keep in mind as you go:
As you weave, you'll also want to keep pushing your work up toward the top. A salad fork is an excellent tool for this!
As you weave, those loose ends might get in your way. You can always twist them loosely together and tape them down to the cardboard with painter's tape.
Then, cut the remainder of the loose end close to your work.
Next, cut each loop, and tie the two ends into a double knot.
Once you've knotted all the loops, you have a secure edge. Do the same thing on the opposite end of your weaving. This edge can act as a fringe, or you can sew this edge to the inside of your work, like a selvage.
That's the process! Now, go enjoy some weaving time.