Wear shoes that are the correct size. They should feel snug at the toes, but not too tight.
Apply petroleum jelly to the foot before beginning to mold the cleats. The process of molding synthetic cleats can result in blisters and the petroleum will help prevent that.
Put on the socks that will usually be worn with the synthetic cleats.
Put on the cleats and lace them up as they would normally be for a game or practice.
Walk around on a variety of surfaces, such as asphalt, sand, carpet or grass. Your foot muscles move in different ways over each type of surface, and this will help to stretch out the synthetic material of the cleats. Do this for about an hour each day until they start to feel more comfortable.
Keep wearing the cleats as you submerge them into lukewarm water for 20 minutes and then allow them to completely dry.
Introduce the new cleats to practices and games slowly. Only use them for about an hour of play at a time.