The strap had a handle that was one tefach (handbreadth) long, and the strap was one tefach wide. It had to be long enough that, when the sinner was hit, its tip reached all the way across his body to the edge of his stomach.
When he was being hit, the person striking him had to strike one third of the lashes on his chest and two thirds on his back, divided between his shoulders.
The sinner was not hit while standing or sitting upright. He was hit while standing or sitting, but leaning over. The Pasuk (verse) tells us that the judge will lower him, so he will be lower. He cannot be all the way down, and he cannot be standing upright, but he is lowered a bit.
The person administering Malkot (lashes) would hold the strap in one hand and strike with all his strength.