If I am starching yardage, I mix a solution of Sta-Flo liquid laundry starch and water, "paint" this on the fabric using a large wall painting brush (fast) until the fabric is saturated, toss the fabric in the dryer, and then iron with steam. Steam will re-activate the starch just enough. I do think it's a good idea to let the fabric rest awhile before tossing in the dryer, as it can take some time for the starch to penetrate all the fibers -- especially if the fabric is not prewashed. I do not prewash fabric, and the finishes on the fabric from the manufacturer can slow down absorption of the starch.
For spray starch, I usually spray the right side of the fabric, try to wait for the starch to absorb (I am impatient, so this step is hard for me), and then iron with steam from the wrong side. Too hot of an iron on unabsorbed starch will scorch the starch. The scorch will wash out, but it's discombobulating to see!
Sizing provides so little additional stiffness and stabilization that I generally don't bother to use it.
In case you are wondering, starch is an organic product (usually made from corn starch, but can be other vegetable starches) while sizing is a man-made chemical product.