Purchase a pair of swimming goggles from your sporting goods store. Be sure to select a size that best suits you (i.e. children will need child-size goggles). If you wear glasses or contacts, you may want to purchase prescription goggles from your eye doctor.
Hold the goggles against your eyes. Pull the elastic band over the top of your head, and allow it to rest around your head, just about your ears. Test the default sizing of the elastic band.
Make adjustments to the elastic band as needed using the buckles on each side. Pull the buckles tight to make the elastic band smaller if the trial fit was too loose. Loosen the band within the buckles to make the elastic band longer if the trial fit was too tight.
Try the goggles on with the newly adjusted elastic bands. Make further smaller adjustments as necessary.
Place the goggles on, over your eyes first and pulling the band around your head second. Adjust the plastic goggles over your eyes, ensuring there is a tight seal against your face before jumping into the water.