My Tips:

• The entrance to the dome is to the right of St Peter’s Basilica’s doors.
  
• We skipped the lines by going first thing in the morning. Check the opening times because the dome is open at different time during different seasons. 
• A Guided Tour is not needed.
• Time to climb takes half an hour to an hour. This includes the dome climb and time at the top.
  
• You can climb 551 steps or pay the extra 2 euros to take the elevator to the rooftop. It is 8 euros without the elevator to the first part/rooftop. From there is only another 320 stairs to the top.
  
• Save walking around the rooftop until after climbing the Dome. Your main goal is to avoid crowds in the narrow stairs to the top of the Dome. 
• I am claustrophobic but had no problem because it wasn’t crowded.
• There are different stairs down, so the traffic is not going both ways.
• The rooftop has a gift shop, a refreshment stand shop, and a bathroom. 
• Be sure to check out the back side of Jesus and the apostles.
• Morning pictures from the Dome of St Peter s Square can be washed out. If you are wanting a perfect picture of the square, go later in the day.