Making place mats with yarn and nails is a similar process to using a ready-made loom with precut rags. You make a frame from wood and nails, then using a thick yarn, you wind the yarn around the loops in a fashion similar to weaving. This produces a nice, thick place mat. Use up pieces of yarn you already have to make a multicolored place mat or start a new skein to create a place mat that's all one color.
Join four 15-inch-long pieces of 1-inch by 3-inch wood together at the corners to form a square frame. Hammer 1-inch long wood nails into the corners to join the pieces together to make a loom.
Hammer nails in every 1/4 inch along the wood. Hammer nails in on all four sides to form the posts, leaving at least 1/2 inch of nail above the wood to wind the yarn around.
Tie the end of a ball or skein of bulky-weight yarn at one nail at the top, left corner of the loom. Pull the yarn tightly and bring it down to the bottom left corner nail. Pass it to the left of the nail, then around the back of the nail, bringing it up to the right of the nail.
Pull the yarn tightly and bring it up between the far left nail and the nail to the right of it, passing it around the second nail and down to the bottom of the loom. Wrap the yarn around the back of the nail to the right of the far left nail. Continue to pass the yarn to the top and bottom of the loom, passing the yarn behind each nail as you go until you reach the top right nail.
Cut the yarn, leaving a 1-inch-long tail and tie it to the top right nail. Turn the loom so the right side is now facing away from you and the nail with the yarn is on the top left side.
Continue bringing the yarn up and down the loom, only this time, weave the yarn in and out of the first strands of yarn, then pass it behind the nails as you did before. You have now woven one place mat.
Cut the yarn, leaving a 2-inch-long nail and form a loop around a crochet hook. Remove the yarn from the top right nail and place that loop around the crochet hook also. Pull the loop closest to the hook though the back loop.
Continue around the perimeter of the wooden frame in this manner, crocheting the edge loops to finished the place mat. Weave the tail of the yarn into the last few stitches to hide it. Create three more for a set of four place mats.