Newly planted corn is a temptation to many species of birds and small mammals, and not just to crows. Ideally, a scarecrow works by providing a human-shaped presence in the garden or cornfield that frightens the wildlife away. While it might work for a while, the critters eventually get wise to the fact that the scarecrow is not a living thing and that is poses no real threat. Adding a noise component to your farmyard creation may be more effective at scaring and keeping away the varmints.
There are no hard and fast rules to making a scarecrow, and you can use whatever you have on hand. The trick is to make it durable, yet with enough loose moving parts, such as flapping sleeves or pants legs, to make it appear to be alive. Use a frame built on a support that you can sink deeply enough into the soil so that it doesn't topple over in the wind, but not so deep that it can't be easily moved. Use durable items to stuff the scarecrow. Old rags, straw, leaves and plastic grocery bags will withstand outdoor elements and last longer. Any old hats will do and change them out regularly to keep the scare-tactics fresh.
Another trick is to relocate the scarecrow to a different part of the garden each day, which gives the illusion of movement. Suspend aluminum pie plates, bells, CDs or hang inexpensive wind chimes from the arms. Use anything that will reflect light and make noise. Change the movable items out on a regular basis so the birds don't get used to them. Use a smooth round pole as the support, and sew flaps of material to the undersides of the scarecrow's sleeves; join them at its waist to act as sails to trap the wind and make it spin around. Move the scarecrow to different locations so the birds don't get used to seeing it in one section of the garden and become unafraid. The more movement and noise you can create, the more likely it is that it will frighten feathered foragers away.