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.