Look up the definition of the word "insular" in a dictionary. As defined by "Merriam-Webster," "insular" means "of, relating to, or constituting an island." It also says that "insular" is "characteristic of an isolated people, especially those being, having, or reflecting a narrow provincial viewpoint." It can also refer to an island of tissue or cells. "Insular" originates from the Latin "insularis" or "insula," which means island, and which was first used in 1611.
Think about which meaning of "insular" you want to use in your sentence. "Insular" is an adjective, which means it can be used to describe someone or something.
Find synonyms of "insular" in a thesaurus that might make it easier to form a sentence. Some synonyms found on Thesaurus.com include narrow-minded, closed, confined, isolated, provincial, restricted or separate. If it helps, think of something that's "insulated," or separate from the rest and hidden away from the world in a confined space.
Form your sentence either using "insular," or one of its synonyms that you can later substitute with "insular." Decide what noun "insular" will be modifying (or describing).
"The Free Online Dictionary" gives examples of how the word can be used: insular territories, insular culture, insular man, insular attitudes. "Merriam-Webster" uses "insular" to describe boarding schools ("the insular world of boarding schools)" and to describe a community ("an insular community that is not receptive of new ideas").
Read your sentence. Substitute "insular" with a synonym, such as "isolated," and see if it still makes sense. If it does, you can feel confident that you're using it correctly in your sentence.