Correct spelling, explanation: skied is a past form verb of ski. The verb is regular, so to create such a form we need only to add the suffix -ed. Therefore skied is the correct form, while skiied is incorrect – there is no need to add another i.

Definition of skied:
verb, past simple and past participle form of ski, to do sport using skis, to use skis
I skied when I was a child, but now I prefer snowboarding.
I have never skied, I don’t like winter sports at all.

Incorrect spelling, explanation: this form is a misspelling because double i is unnecessary. Skied is the past form of the verb to ski, and it is a regular verb, therefore we only add -ed ending to it. No other extra letters are necessary for this care, so skiied is incorrect, and skied is the correct form.