Wikipedia has great stuff on the matter:
"Here the executioner gave him the first stroke. His cries were terrible. "O Jesus! Jesus, have mercy upon me!" This cruel scene was much lengthened out, and of the utmost horror; for as the headsman had no skill in his business, the wretch under his hands received upwards of fifteen several blows, with each of which were intermixed the most piteous groans, and invocations of the name of God. At length, after two strokes given on the breast, his strength and voice failed him. In a faltering dying tone, he was just heard to say, "Cut off my head!" and the executioner still lingering, he himself placed his head on the scaffold: in a word, after four strokes with an hatchet, the head was separated from the body, and the body quartered. Such was the end of the renowned Patkul: and may God have mercy on his soul!"