Draw the slash lines on front and back to give the extra volume.

Drop the armhole by 2-3cm. on center front, draw the placket until the hip line for the polo buttoning.

Get the basic sleeve, measure the sleeve cap and change it (drop and widen as the arrows show) until you match the measurement of the new armhole.

Draw two horizontal slash lines (one for the cuff and one above the elbow).

From the sleeve cap to the first horizontal line, draw 3 slash lines to add the gathering.

Get the front and back pieces and spread the two sections by 12-16cm.

Redraw the shoulder line and raise its top by 1,5cm.

Spread the sleeve pieces by 4-5cm and raise the cap by the same amount.

Finish drawing the other pieces: left and right placket, different for the hidden buttons, two pieces collar and cuff.

Add front, back and sleeve copied from the previous step.