Add Custom Fields To Product Page Of Shopify Site

I own a custom dress website which custom makes dresses to customers unique measurements (bust, waist, hips, shoulder to floor, hips to floor length). My website address is mydebdress.com.au. I need you to make a form which pops up when a customer clicks to purchase a product which asks them to enter their unique measurements for the dress. The fields will include the 5 things mentioned above aswell as a field for additional requirements. I also need there to be space within the form but below the fields for some tex explaining how the customer is to measure themselves. This is all within the shopify system so you must know how to use it.