A way to practice is to place your lips together and teeth apart. Open your lips slowly while keeping your teeth apart and there you have it. Another important aspect of fellatio is lubrication. Most people use their own saliva as lubrication. If you are using a condom for fellatio, try to buy an unlubricated variety.
Lubricated condoms tend to have a bad taste.
If you d like to try lubricant during fellatio, you can purchase a flavored lubricant to turn a normal latex condom into something tasty, or use it on its own for a unique experience.