The lengths of framing for this project are measured from the exterior side of the mitre, or the maximum length of each piece.
To frame a larger or smaller picture, simply take its measurements and subtract 5mm so the frame will slightly overlap the edges of the picture. Make the first mitre cut, then mark the length on the interior side of the frame edges.
A tenon saw is a type of handsaw used to make precise cuts in timber. It has a reinforced back to make it more rigid, and finer teeth than an average handsaw, allowing it to make neater cuts.