Tim,
With your water separator, you drain the fuel out of the bottom of the plastic bowel, then unscrew the plastic bowel from the filter, then remove the filter from the base mount.
The little 1" oring goes on the threaded center base mount and the other two big Orings go on the filter, one on the top to the base and the other to the plastic bowel to the filter bottom. Lube the Orings with a bit of oil.
Going back together, screw the bowel onto the filter, fill the filter with fuel and screw it onto the base. Sometimes these can be a pain to stop them from leaking and sucking air. When you get it back together start up the Cat and watch the bowel to see if it stays full of fuel, if it starts to empty you're sucking air. Shut off the motor and tighten up the assembly some more.