The sentence "スキーはどこでしてたの？" translates to "Where were you skiing?" in English. Let's break down the grammar:
スキー (sukii): This means "skiing" and is the topic of the sentence.
は (wa): This is the topic marker. It indicates that "skiing" is the topic being discussed.
どこで (doko de): "どこ" means "where," and "で" is a particle indicating the place where an action occurs. Together, "どこで" means "at where" or "where."
してた (shiteta): This is the past continuous form of the verb "する (suru)," which means "to do." "してた" implies an action that was ongoing in the past, so "してた" translates to "was doing" or "were doing."
の？ (no?): This is a sentence-ending particle that adds a questioning tone, often used in casual speech. It softens the question, making it sound more informal and conversational.
So, the entire sentence "スキーはどこでしてたの？" asks about the location where someone was skiing, with a friendly, informal tone.