[Actually, it wasn't - sewing a wound on a standing person would be awkward even for a grown adult. But, fair enough: wasted opportunity for a jab.
Sebastian obediently takes a seat on the stool with a "yes, my lord", and after considering which injury makes for the best training-wound, unbuttons and shrugs out of his shirt enough to reveal the gaping shoulder wound.]
Normally you would have to make sure the wound is devoid of any kind of dirt that you would not have the wound closing around, but- [he shrugs the shoulder in question (it bleeds more)] as you can see, it is clean enough already.
[And useful lesson or no, Sebastian is annoyed at life in general and would like this not to drag out.]
Doing something like this on a human and forgoing anaesthetics would be ill-advised. Ether is commonly used, but the dosage can be a delicate matter. Even I, were I concerned about not accidentally killing the person, would leave the administering of it to a physician. Inebribation, though more crude, is the safer method.
[He makes a small dismissive gesture with one hand - Ciel knows how to get someone drunk already without practising on Sebastian.]
But, speaking of alcohol, it too works to clean a wound. The purer the alcohol, the better, although even drinking wine will do if nothing else is at hand - then you would choose whichever bottle has the highest alcohol content, and apply it only after you have rid the wound of dirt. I keep a bottle for disinfection in the kit. Pick it up and then pour some of it in the wound.
[If nothing else to at least practice something of the pre-work and not just have it be Sebastian Covers The Theory Hour.]