A fan on its own doesn't lower the temperature of a room. It just keeps air moving over your baby's skin, which is what actually helps them feel cooler. That's why the fabric touching them matters more than the number of layers. Muslin is woven loosely, so air passes straight through it instead of getting trapped, which is why one loose piece, a bodysuit or romper, is usually enough even on the warmest nights. If you're not sure how much to put on, our answer on how many layers a baby needs in hot weather walks through it room by room.
Set the fan to circulate air across the room rather than blow it directly on your baby. Keep it a few feet from the crib so the airflow stays gentle, not a steady blast on one side of their body. A fan makes a room feel cooler than the thermometer says, so don't rely on it alone to judge comfort. Feel the back of your baby's neck or between their shoulder blades: damp and hot means take a layer off, cool and dry means they're fine. Our guide on what temperature a baby's room should be in the tropics has more on reading the room itself, fan or no fan.
If your baby sleeps in a sleep sack, a fan-only room usually calls for a lighter sack and less underneath it than you'd use with air conditioning, since the moving air already does some of the cooling work. We cover that combination in what to dress a baby in under a sleep sack in the heat. Wash new muslin in cold water on a gentle cycle before the first wear, skip the fabric softener, and it will only get softer and more breathable with each wash. Some creasing after washing is normal for muslin and doesn't affect how it wears.