Inletting the entry pipe is, for me anyway, one of the dreaded tasks of building. I cut the slope at an angle approximate to the slope of the skirt of the pipe. I also round the forearm before starting. Make sure your tools are razor sharp before starting. Use your ramrod to make sure the bottom of the pipe is deep enough for it to pass through.
There are many here more experienced than I am. Go slow. G'luck.