Positions for spanking (1) 

The first is the traditional position for spanking on the buttocks, also known as OTK (on the knees) and it consists on positioning the bottom not actually on your knees, but on your lap. All of the spankee's weight should rest on the spanker's legs. The left hand of the spanker holds the far side of the waist of the spankee, and the elbow rests on the middle of the back, for preventing the spankee to rise. The culprit feet should barely touch the floor, and the hands can just hung, or the left hand can hold the chair for stability, and the right can be held by the top's left hand in the small of the back of the bottom. (Of course, being the punisher left handed, all the positions are inverted). This is the position for spanking with the hand or short instruments, like a slipper, hairbrush, paddle, etc. 

This is a variant of the former, used for a spankee that moves too much or is too strong to be held in the traditional OTK position. The punished bends over the left leg of the punisher, whose right leg is used to press over the spankee's legs to hold them down.  

Another variation of the OTK is lying on a couch. This is a good position for sensual spankings, because the spankee is comfortable and can concentrate in the feelings on his/her buttocks. Can also be used on a bed, (but mattresses are usually too soft for a good stability), or in an emergency, on three chairs side by side.

Another, and probably my favorite. The punished is comfortable enough, but s/he is also restrained and cannot free him/herself, so s/he must not bother with self control. So the punished is able to wriggle and to move the buttocks, but cannot avoid the punishment. Which is good for the spankee, and a joy to the spanker. You can also use two chairs.

Another possibility if the punished is strong or moves too much is sitting on a bed, and holding the punished under the left arm, with the head back.