The easiest way to measure your head for either a Z-1 or an Evo is to take a cloth tape or a sturdy string and find the circumference of your head above the ears at the widest point.
SMALL: 21 5/8 - 22 inches; MEDIUM: 22 1/4 - 22 5/8 inches; LARGE: 22 7/8 - 23 1/4 inches; X LARGE: 23 3/8 - 23 7/8 inches; XX LARGE: 24 - 24 3/8 inches.
Put one thumb under the chin portion of the liner and your other thumb at the center in the back. While looking straight ahead, pull the helmet down over your head. The helmet portion should end just under your lower lip and the neck attachment should be snug under your chin. Tighten the bungie to fit your head and stow the rest of the elastic in the hole in the back. DO NOT TIGHTEN THIS EVERY TIME.
Although your lens has an anti-fog coating on it, warm air on the inside applied right next to the lens (from your nose or your mouth) will cause fogging. The problem is usually easily corrected by a simple adjustment of the liner -- disconnect the liner under the nose and reattach it below the actual helmet (visor) opening. This allows better air flow and typically eliminates the problem.
WASHABLE LINER -- Your liner is washable. Just put it into a laundry bag (or a pillow case) and put it into the washing machine. Air dry.
REMOVING MARKS -- Most of the marks on your helmet will be color added, not taken away. Clean your helmet with water and just a bit of Murphy's Oil Soap and a soft cloth. More persistent marks may be removed with a diluted bit of Soft Scrub with light circular rubbing motion. You should try this on a small area first (like under the visor) and see how it goes. You can touch up gloss helmets with "Testors" paints made for model cars. Magic markers in the same color are also good for a quick fix, particularly on the soft touch finishes. Soft Touch finishes look like new when cleaned with Soft Scrub and a soft cloth.
HOW TO CLEAN THE Z-1 LENS -- Use a very soft cloth and clean water for this project -- no glass cleaners or abrasive fabrics! The lens has an anti-fog coating on the inside and an anti-scratch coating on the outside that will be damaged if cleaned by any other method.
HOW TO CHANGE OUT THE Z-1 LENS -- The lens on your visor is changeable and replacement lenses are available in clear, smoke, and yellow. Take the visor off your helmet with a quarter outside and a half inch wrench inside. The original black plastic bolt is a quarter-turn locking mechanism so pay attention to how it works -- it locks in one position and releases in another and requires popping it off the first time. Now take the old lens off. DO NOT REMOVE THE TAPE OR THE PROTECTIVE COVERINGS OFF YOUR NEW LENS EXCEPT NEAR THE HOLES BOTH FRONT AND BACK. Install the new lens and visor back on the helmet (the lens should move independently), but do not yet tighten the bolts. Now remove the tape and the protective film on the front side and adhere it to the visor from the center out (this allows equal stress across the lens). Now tighten the bolts. Finally, remove the protective film from the back of the new lens and you are ready to go.