Selecting an exterior mount makes the most sense if your frames are less than 3–4 inches (7.6–10.2 cm) deep. To find out if your frames are rectangular, measure diagonally across your window frame with a tape measure. First measure inside of the window frame from the top-right corner to the bottom-left corner. Then measure again from the top-left corner to the bottom-right corner. Compare the 2 numbers to figure out if your window is square.  If the 2 measurements differ by over 1⁄2 in (1.3 cm), your windows aren't square and you need to use an external mount. Exterior mounts are helpful if you would like to make a window appear larger than it really is. An external frame is also the best option if you have walls made of a strong material, like wood, brick, or concrete. Also use an exterior mount if your window frame is worn or splintering and would like to cover it up. The headrail is a long metal upper piece of the shade that the shade's fabric comes down from. Take the length measurement of your headrail and of the top of your window frame. Find the midpoint of both distances, and center your headrail above the window frame. Mark the location of both endcaps of the headrail on the wall with a pencil. Large Roman shades used in an exterior mount may have multiple metal brackets to support the shades' weight. In this case, the additional brackets will be spaced evenly within the space between the 2 edge brackets located 3 inches (7.6 cm) in. Mark these locations on the frame or wall as well. These points are where you'll center the 2 brackets that hold up the Roman shade. Use your tape measure to measure 3 inches (7.6 cm) in from each side of the measured length of the headrail. Then use a pencil to mark both of the 2 locations on the wall above your window frame with a small “X.” If your Roman shade headrail comes with multiple loose brackets, space them evenly over the top of the shade. With an exterior mount, you could be dealing with either of these types of materials. Since plastic and drywall are a little brittle, drilling directly into them could fracture the material. To drill a pilot hole, drill a hole through the wall above the frame using a 1⁄8 in (0.32 cm) drill bit. Each of the 4 pilot holes you drill should be 1 inch (2.5 cm) deep. These screws and fasteners should be included with the Roman shade when you purchase it. If not, you can purchase them from a hardware store. These materials are dense and sturdy and a regular steel drill bit won't be able to penetrate them. If you are mounting your exterior Roman shades on one of these substances, you'll need to use a 1⁄8 in (0.32 cm) carbide drill bit drill the pilot hole. Hold the drill so the bit is at a 90° angle to the wall, and drill the 4 pilot holes each 1 inch (2.5 cm) deep. Carbide is a type of coating that manufacturers put on the tip of steel bits to make them carbide bits. Carbide bits are sharper than ordinary steel bits, and will retain their sharpness even after drilling through concrete or brick. Place 1 of the brackets in position so that its 2 holes line up with 2 of the pilot holes you just drilled. Set the tip of a screw in each of the holes. Then, using your screwdriver, turn the screws clockwise until they're held tight against the wall. Repeat the process with the second bracket. At this point, you're almost finished!  Unless parts are missing from the shade-installation kit, you shouldn't need to purchase any additional brackets or screws. If you are installing your Roman shade in material other than wood, you will need different types of screws or fasteners. For example, you will need to use hollow wall anchors or toggle bolts if you are hanging Roman shades in plastic or drywall. For exterior-mounted Roman shades, the headrail should have 2 indentations where it snaps onto the brackets. Angle the headrail upward and push the top into the brackets, then pull downward until the headrail snaps into place. This should secure the headrail (and attached shades) in place.  In some sets of shades, you may need to snap the headrail into place first and then snap the shades into the headrail once it's in place. To remove the headrail at a later date, tilt the headrail downward and give it a sharp tug out and away from the wall. Give the cord a tug to verify that the shades are secure and functional. If the shades come away from the wall when you pull and release the cord, check to make sure that all screws and fasteners are tightened. You may also need to snip off small plastic loops holding the shade in its “closed” position. If you've opted for a set of cordless Roman shades, you can raise and lower the shades simply by grasping the handle behind the shade and raising or lowering it to the level you'd like it to be at.

Summary:
Use an exterior mount if the window frames are rectangular or shallow. Measure the headrail and mark the locations of the endcaps on the wall. Mark 2 points 3 in (7.6 cm) in from the ends of the headrail. Pre-drill pilot holes if you're installing shades onto drywall or plastic. Use a carbide drill to drill pilot holes in concrete, stone, brick, or tile. Screw the brackets into the pilot holes with a screwdriver. Attach the shades' headrail to the wall by snapping it in to the brackets. Pull on the cord that adjusts the shades to raise or lower them.