When you do something inexpertly, you do it without much skill. The work looks clumsy, or the result is a bit wrong. Like inexpert, it is a formal, slightly bookish adverb. In everyday talk, people say “badly,” “clumsily,” or “not very well.” Badly can sound harsh; inexpertly is softer — it points at the skill, not at the person as a whole. It tells you how the action was done, not what kind of person someone is.