Hold the cap in front of you upside down with the o-ring groove facing up. Place one end of the o-ring into the cap groove at the point closest to you. Hold o-ring down with the thumb and forefinger of one hand. Take hold of the far end of the o-ring with the thumb and index finger of the other hand. Roll o-ring slightly back towards you as you stretch the o-ring into the remainder of the cap groove. (See manual for diagrams)
Lubricate o-ring regularly with silicone lubricant provided. DO NOT USE petroleum-based lubricants!