Cut slits in the top and bottom of the material (the pieces that aren't stuck to the plant pot). Turn the pot upside down and cover the bottom of the pot in a layer of mod podge. Then start folding down the slits, applying more mod podge when necessary. Leave to dry.
Once dry, turn the pot the right way up and use the same technique to fold down the material along the rim of the plant pot. It doesn't have to cover the whole of the inside of the pot - just enough so that the pot can't be seen when the plant is inside.