You get someone who is good with wood to turn one end of it into a stake shape (perhaps a friendly vampire slayer would do it for you). I got Billy Steve to do it.
Then you fit the head onto the other end and depending upon how you want the arms this is what you do next. If you want traditional sticky-out arms you simply nail a slightly narrower piece of wood to the upright one positioned just under the chin. Because my scarecrow is a bride I decided to have her arms down so I put a nail in the wood just under the chin and rested a wooden coat hanger on it, then I secured the coat hanger by tying a bit of garden string around it.
Then you hammer the upright into the ground.
I stuffed some white gloves with polyester stuffing so that they looked like hands and then sewed the gloves to the end of the sleeve of the old wedding dress I planned to use.
Next I put the top part of the dress onto the coat hanger, I had split the dress into two sections, top and skirt, because I wanted to use some of the material for something else. I closed the buttons at the back leaving a small gap for pushing stuffing through and ran a few very rough stiches along the waistline of the dress securing it around the upright post.
Then I stuffed the top part and sleeves with straw (I bought a very cheap pack of compacted straw intended for use as small rodent bedding). Next I sewed up the small gap in the back which I had left open for stuffing.
I then stitched the skirt piece very roughly, gathering it up as I went along, to the bottom of the blouse bit and tied it off tightly.
Lastly I pulled some ivy down from the fence and tied it together with a bit of ribbon, placed it in her hands and put a couple of quick stitches in her gloves to hold them together. (If you cant sew at all you could do all the stitching with safety pins and I am sure it would work just as well).
Then I raised my hands in the air and very dramatically called out in a loud voice "Oh Goddess of the garden, please make all my vegetables grow!"
"Now you are being silly" said Billy Steve.
"No" I replied "This is being silly!"