Dry fit the new piece into position (don't attach yet). It should be flush with the existing ceiling, rest on half a ceiling rafter on each side, and have a slight gap around the edges. Pic 1.

Trim the new piece or the existing ceiling edges if you need more room on a rafter to affix it securely.

Dry fit again after trimming. Use the framing square and pencil to draw a line from the trim arrow to the ceiling arrow you marked in Step 4. Pic 2.

Bring the new piece back down until you have placed your screw gun and drywall screws in easy reach of your position on the ladder before you begin the installation.