Hi Jessdog,
All I have ever done after emptying the cassette is add some water (about a litre) to the cassette at the dump point then back at the van before inserting the cassette add the mix you describe spoon by spoon straight into the cassette, shut the gate and give the cassette a shake then slide her in, all done.
All the mixing is done on the outside of the van in the cassette before the cassette is inserted.

One note: be very careful with the eucalyptus oil as it will dissolve the cassette top slide cover if you accidentally drop some but the cassette tank plastic can handle the oil.

JR