Installing rounded-top windows is easier than most people realise. In fact, if the framing is in place for a rounded-top window due to a pre-existing rounded-top window having been installed, the process is as simple as installing a normal window. However, if you need to create your own frame for the rounded area of your new window, the process is more complicated. You will have to create templates, which requires proficiency with various tools, especially saws. The average homeowner can do it.
Trace the rounded-top window onto the poster board with a pencil. Cut out this template with a pair of scissors. Tape the template to the roofing felt. Mark the opening to prepare for cutting.
Create a hole for the reciprocating saw to cut the opening. Cut so that the opening will be ¼ inch bigger than the rounded-top window. You can then measure for the framing piece by placing your square against the stud and measuring toward the header area from the outer edge of the stud.
With a circular saw, cut the framing piece with a bevel of 45 degrees at the left and right ends. Nail this piece into the window opening after checking the accuracy of your cut.
Remove the window siding. Apply liquid flashing and allow it to dry according to the manufacturer's instructions. You can apply this flashing with a brush, but you can also applied it with a liquid flashing gun that comes in the liquid flashing kit. Liquid flashing usually takes between 24-48 hours to cure.
Install the rounded-top window. Use a compass to mark the new siding. Use a jigsaw to cut the siding and make adjustments with the compass. Scribe where necessary.
Install the siding by caulking along the area where you intend to install the siding. Hammer in 6d galvanised nails to secure the siding and then caulk around the entire window edge. Smooth the caulking down with a scrap painter's mixing stick and clean up the surrounding area with a small damp rag. Let the caulk dry for 24-48 hours before using your new rounded top window.