Measure the door opening's length and width. Add 2 to 3 inches to the width so the felt does not hang completely flat when closed. Transfer this measurement to a piece of felt. Draw light pen lines down the felt to mark the outline, using a yardstick as a flat edge.
Cut out the felt with a sharp scissors. Apply a thin bead of fabric glue to the felt's cut edges, to prevent fraying, and allow it to dry.
Mount a rod either above the door opening or within the door opening, following the manufacturer's directions. A curtain rod would work well for above-door mounting and a shower curtain rod works well if you want to mount it inside a door space.
Measure 1 inch down from the felt's top edge. Lay a yardstick across the felt at this location. Place a small mark 1 inch in from either edge. Measure the distance between the two marks and divide by 4. Place more marks at these locations between the two marks. For example, if your felt is 48 inches wide, the distance between the first two marks is 46 inches. Divide 46 inches by 4 to get 11 1/2. Place a mark every 11 1/2 inches between the first two marks.
Punch a hole through the felt at every mark along the top edge, using a hole punch. Apply a thin bead of fabric glue around each hole to reinforce the felt. Allow the fabric glue to dry. Insert a curtain clip into each hole and hang the clips from the curtain or shower curtain rod.