Installing a wall thimble requires measuring and cutting holes for both the interior and exterior thimbles, and then screwing the wall thimble into place. For most houses, the only tools this job requires are scale dividers, a pencil or chalk, a reciprocating saw, and an electric drill or screwdriver.
Always install the interior wall thimble before the exterior one. Using the scale dividers, measure the largest diameter of the interior wall thimble. With this measurement, use a reciprocating saw to cut the circle out of the interior wall.
After cutting out the interior circle, drill a small indicator hole in the exterior wall around the middle of the circle. Once outside, cut an identical hole into the siding and exterior wall. Center the exterior wall thimble over the hole, and press it against the siding. Wherever the wall thimble contacts the siding, make a vertical mark. Use your reciprocating saw to cut slots into the siding at each mark. Do not cut into the exterior wall; only make slots in the siding.
After cutting the slots, fit the exterior wall thimble into them. If the thimble does not fit well, adjust the size and location of slots, making cuts wherever necessary. After ensuring a good fit, secure the wall thimble to the wall using screws.