Be gentle when doing a water change. You don't want to stress your fish. Goldfish are very curious, however, and over time, you will find that your fish won't mind your hands in the tank at all. Before beginning, make sure your hands are clean and well-rinsed. There should be no soap or detergent on your hands at all.
Soap is poison to fishes.
Also, disconnect anything electrical that may be nearby. Since your fish are used to the water quality of your tank, adding too much new water can stress them. Small, frequent water changes are best. If your tank has algae you may want to scrape it off before beginning. You can use an algae scraper or an old credit card.