As a rule, unless I'm doing a four-panel style, I place boning about every two inches at the smallest part of the waist. Boning is also placed to either side of the lacing eyelets. It can be inserted into casing stitched either on the inside or outside of the corset, or inserted inbetween the outside layer and the lining layer on the conventional method. If the corset is intended for wearing underneath clothes, inside boning will be more discreet. As I usually use the alternate construction method I prefer to either topstitch the seam and stitch the allowance down to create one channel next to the seam, or to press the seam allowances open and stitch down either side to create a channel on each side of the seam.
Some people recommend always using spiral.
It is better for seams which curve to the side, but that is the only time I use it, as the ends tend to catch in the casing and can be impossible to get free. It also isn't quite as sturdy as solid steel. If using quarter inch boning on a four panel corset you will probably want to put one strip of boning to either side of the seam. You should always use the solid boning next to the lacing eyelets, and it is better than the spiral for the side panels as it gives better support. If doing outside casing you can use same or contrasting fabric, but make sure it's heavy fabric, folded into thirds.