Tear strips about 1-1.25 inch wide by 14 inches long. I used about 8 strips for the horizontal part of the placemat. 

Then tear strips for the vertical section. I ended up using about 11 but that may vary a bit so make sure you have a few extra.

Lay them out in varying shades or any pattern you like.

Then weave the shorter denim strips into the longer ones. Over and under. Once you have a few in place, it is easy to just lift every other piece to the side and then set your vertical piece right in.

Next, use a brush and some fabric glue to glue the edges in place on the front side.

Then flip it over and glue the back edges.

Add some shelf liner to the back with more dots of fabric glue.

Then trim any excess away.