Loose weave natural burlap manufactured by the DeWitt company, www.dewittcompany.com and sold at many garden centers and large chains such as Lowe's. Rolls are 3' X 24' and sell for less than $10.
The loose weave makes it easy to shred especially when cut on the bias. Avoid the temptation to cut with the grain in short little rectangular pieces.
To cut on the bias, begin by cutting off one corner.
Cut strips, noting the strands need to be coming diagonally from both sides. The last strip can be cut up a little further but as you approach the edge the strands stop coming diagonally.
Where the strands are no longer diagonal, cut across.
Turn the part where strands are not diagonal and cut the corner off like in the beginning.
Continue cutting long strips on the bias.
To shred, start at the strip end and pull in the center of the end so that strands come from both directions.
It goes pretty fast when you cut on the bias and use the loose weave burlap! I use to buy burlap at the clothing goods store and it was a tight weave that was designed for bulletin boards. I would make my fingers sore struggling with shredding it and it would take me hours! It was such a monumental task that my friend Doyle Johnson, put shredded burlap on his Christmas Wish List!
It can be used without washing and makes a nice woven nest. Natural burlap does have a slight smell but soon airs out.