Take a picture of the arch on the wall. Measure the length of the arch from one side, where you want the drapes to start, up and over the arch to the same point on the opposite side. Measure the distance from the ceiling to the top of the arch. Record these measurements. Put the photo into a clear plastic binder sheet protector. Planning the placement of the knobs is easier, and more accurate, if you use a photograph.
Plot the knob placement, marking on the sheet protector with a dry-erase marker, equidistant around the arch. If the drapes are premade and pleated, plan on one knob per pleat. If the premade drapes are flat along the top, plan on one knob for every 12 to 14 inches of flat fabric plus 1. If the flat drapes are 60 inches wide and are to be hung on one half of the arch, which measures 30 inches, plan on 5 plus 1 to equal 6 knobs on one one side of the arch, the other side of the arch has 5 knobs. This allows for a knob for every 12 inches of fabric plus one at the apex of the arch. Decide the placement of the knobs above the rod, referencing the measurement of the space available to the ceiling. This placement is arbitrary.
Divide the drape into the appropriate number of sections and mark the knob placement on the top edge of the drape with a pin. Decide how the drapes will be attached to the knobs. A fabric loop or tie, sewn to the drape at each knob placement is a common attachment. Make these loops or ties of fabric or ribbon, and make them long enough to slip over the knob if a loop. If a tie, ensure there is enough extra to tie easily over the knob. If the drape is pleated, the drapery pin rests in the bottom of the loop or tie and the tie or loop is not sewn to the drape. A decorative wooden ring, placed on the knob parallel to the wall, allows a drapery pin attachment through the eye screw of the ring. If the ring has a clip rather than an eye screw, the top of a pleated or flat panel can be easily attached.
Insert the knobs into the wall at the premeasured locations. If using a ring as well as the knob, ensure that the ring fits over the knob or put it in position before attaching the knobs to the wall.
Loop the ties or loops over the knobs to complete the installation.