How do you think one does it? What event do you think would be most appropriate? What is the basis for populating the dependent list? Are the lists in the dependent defined within ranges on a worksheet? Or as arrays, existing only in code?
Once you work your way through these kind of questions you can put together a starting form. Then, when you get stuck, show us what you came up with and we can nudge you in the right direction.
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Heya, Smitty.