Burlap is a coarse, brown fabric used traditionally to make sacks. It a flexible fabric that does not have much stiffness of its own. If you have some burlap that you want to stiffen, you can use commonly-available spray starch to do the job. Starch is a traditional (though now less-used) way to add stiffness to fabric. You can find spray starch at any hardware store. The added stiffness will allow you to shape the burlap more precisely for use in crafts or costume garments.
Spray the burlap with starch until saturated.
Place an old sheet on the ironing board to absorb excess starch and to prevent creating a dark spot on the ironing board.
Press the burlap with the iron on the "Cotton" setting. Do not iron in a rubbing motion; instead, press directly down onto the fabric. Allow the starch to steam but not to burn. The starching process is complete when the burlap is dry and stiff.
Use a diluted spray during pressing to add extra starch to the burlap. This will result in a stiffer fabric.
Do not store starched fabric; use it right away. Starch is a food sugar that can attract pests.