Don't worry if the cut is jagged, or in case you cut out more than required by mistake.Just push in M-Seal/Suguru and sand it out later. If the windows the you cut happen to be too large, make them smaller again by applying the compound along the edges. it can be sanded out later.
Also make sure you cut out plastic to make way for the charging pin. Pack any empty spaces with suguru/m-seal . (So that the phone is secured in place & doesn't shift or move) (2nd Pic)