Get some wood the height you want the body to be, cut some for legs and arms.
Get some screws and a drill
Screw arms to the body at your wanted height and the same for the legs.
Find something you want as the head and screw it on to the top of the body.
Get some old clothes and put them on the scarecrow.
Cut a piece of wood to screw on the back for a stand.
Screw on your stand.
Make sure it stands up straight.
Find somewhere to place it.