do you have a compass? if not, you will need one. using a ruler or tape measure open the compass to the desired radius. Using a square mark your center on the stock.
looking at your picture above, say for the 3-3/8 radius, from your finshed line come down 3-3/8 and from the end come over 3-3/8. There is your center point. put the point of the compass on that center mark and draw your radius. do the same for each of your curves. Cut out your curves with a jig saw or band saw next to the line then sand to the line.
Welcome to the family!!