There are daily flights from Athens to Mykonos Airport (JMK), and it takes about 45 minutes. If you are in Santorini or Crete, you can take a ferry to Mykonos.
I recommend taking a guided tour. The local guide is an expert on the island and knows exactly what to do in Mykonos in a day.
There are many great places, but I recommend staying in the Old Town (Chora) area. It’s centrally located and close to all the main sights and attractions.
The best time to visit Mykonos for great weather is from late April to early November, when the weather is pleasant and sunny.
Since you’ll only be spending one day in Mykonos Greece, the easiest way to get around is by taxi, bus, or foot.
If you’re short on time, one day in Mykonos is enough to get a taste of what the island has to offer. But spending a few extra days is worth it if you can swing it.