First, simplify your outfit. Clean lines, excellent fit and well-kept shoes look more convincing than a collection of visible labels. Borrow or rent for a rare formal event if needed.
Second, arrive informed. Know the venue, the dress code and the basic subject of the event. Preparation replaces much of the anxiety people mistake for not belonging.
Third, be warm to everyone. Good manners are visible in how you treat staff, not in knowing an obscure fork. Ease, discretion and genuine curiosity travel well in every social circle.