Makkos, Chapter 3, Mishnah 12. The Mishnah explains how lashes are actually administered: how the one receiving lashes is flogged.
Binding the one receiving lashes to the post:
Both of his hands are tied to a post, one on each side. The height of that post reaches roughly to the level of the heart, in the middle of the chest, and he leans against it and bends over it with his hands tied.
Exposing the flesh:
The chazan haknesses - the attendant who serves the court - takes hold of his garment: if it "nikra - nikra" - if it tears, it tears, and if it comes apart along the seams - "nifram" - it comes apart, "ad shehu megaleh es libo" - until he exposes his heart. In other words, he must uncover his chest and his back, so that the lashes are given on his flesh and not on his garment.
The position of the one administering the lashes and the strap in his hand:
Behind the one receiving lashes there is a raised stone platform, and the attendant in charge of the flogging stands on it. In his hand is a strap made of two kinds of leather:
A strap of calf leather - folded over, "echad lishtayim ushtayim l'arba" - one into two and two into four, so that in practice it is a strap of four layers.
Two straps of donkey leather - very thin straps, which go in and out, above and below, within the folds of the calf leather strap. What this means: holes are punched in the doubled calf leather strap of four layers, and the two donkey leather straps are woven through them, in and out.
In summary: In this Mishnah we learned the order of preparation for lashes - tying the hands of the one receiving lashes to a post that reaches the level of the heart, exposing his chest and back so that the lashes are given on his flesh, the attendant standing on a raised stone platform behind him, and the structure of the strap: calf leather folded into four, with two thin straps of donkey leather woven through it.