Hey there,
There are two o rings that are included with your P18 to accomodate different length 18650
batteries. If you are using a longer battery such as the
AW 2600 mah then you would only use one o ring. If you are using a shorter battery, such as the
IMR 18650's then you would use both o rings.
The o rings are installed into the atomizer end of the tube and pushed down to seat on the lip at the switch end. I usually just put it in a little ways then put a battery into the unit and kinda bounce it up and down to seat the o ring all the way.
The washers are to adjust the throw, or how far you have to push in the switch, to get the unit to fire. They are installed under the contact on the switch. You can remove this contact with a screwdriver to install them.
For my P18 I have to use two washers when I am using the IMR 18650's or it won't fire at all, and I use one for the AW 2600 mah.
Hope this helps
Jim