Answer: I'm assuming that when you say "large yucca plant" you're referring to a "mother" plant that has created many offshoots. Spring, just at the outset of a new growing season, is a good time to divide them. Start on the perimeter and work your way in. Remove soil around the base of an offshoot and separate it with a sharp flat spade, or make a clean cut with a sharp knife. Try to get as much of its root as possible, but leave the main root whole.
Plant the offset in a well drained, sunny spot. Be careful to plant it no deeper than it grew before. Water well at transplanting, but then let the soil dry between waterings. In my experience, yuccas must have soil with excellent drainage, but don't demand much else. Good luck!
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