Correct spelling


Correct spelling, explanation: this form is correct because skied is the past form of the verb ski, which is a regular verb. To create past form regular we usually add -ed at the end of the word. Ski ends with i and there is no reason to add anything else, so we simply add suffix -ed, as skied is perfectly pronounceable.

Definition of skied:
verb, past simple and past participle form of ski, to use skis, to travel with skis and/or use them for sport
-What did you do during your winter holidays? -Well, we skied a little, but mostly we went sightseeing.
John skied for the first time yesterday and he loves it.

Incorrect spelling


Incorrect spelling, explanation: as there is no need to add extra i while creating the past form from the verb ski, skiied is an incorrect form and the correct one is skied. Ski is a regular verb and we simply add -ed to it to make a past form. Therefore skied is correct.