A topic sentence presents the main gist of the whole paragraph or article. So, if you want to talk about building a snowman, your topic sentence could be about why building snowmans are of relevance. Then, your supporting details could now include steps or advices on how to build it.
A good topic sentence I could think of is:
During the yuletide season, building a snowman is a good activity to do with family and friends because it promotes team-building and fun interaction.