WEARING gloves & safety-glasses:
Carefully-remove the surrounding-glazing & the old-broken-glass-pane.
Measure the opening (horizontal, vertical, & both-diagonal) dimensions;
mark & score the new-glass-pane a little (1/8"-1/4") smaller than the opening;
carefully break the new-glass-pane on the scoring.
Use glazing-points & glazing-putty to center & secure the new-glass-pane in the opening.