There is an outer and an inner part. To install, you need to remove the outer part. Hold the 510 connector, the part that screws into the ProVari, with two fingers. Turn the outer cap counter clockwise. The parts should seperate into two pieces. The inner part has a flange with two sides flattened.
Start screwing the inner part onto the ProVari, After a few turns you will need to use an adjustable wrench (or regular wrench) to fully tighten it down (that's what the flanges are there for). If you have done it correctly, there will be a millimeter or less gap between the bottom of the inner Hippo attachment and the top of the Provari.
Screw an atty on and check it. If you get an error message, you haven't tightened the Hippo down enough. Once you have contact, remove the atty and screw the outer sleeve on. Take care not to cross thread it. Replace the atty, twist the outer sleeve until it contacts the bottom of the atty. Back it off a quarter turn. Take a puff, and turn the sleeve until you get the desired airflow. Hope that helps.
Note: When you first receive the hippo, the outer and inner part may be difficult to separate. If this happens, use a piece of cloth to protect the finish and grab the outer ring with a pair of offset pliers. rotate the outer ring counter clockwise while turning the inner part clockwise. You may need to hold the inner part on the flanges with a pair of needle nose pliers as well. You will only need to do this one time. The more you take the Hippo on and off and apart, the smoother the threading will function.