Insert the base into a carboy cap or rubber bung, insert the "cup" piece upside down into the base, fill to the line with sanitizing solution (or vodka), place cap on top.
Like this:
There's a line on the outside of the base that I usually fill it to. The most important part is to fill it so that the liquid is higher than the holes in the smaller cup part, otherwise it won't really accomplish much.
Econo lock air lock
Linear Mode
