Seemed easy to me too, just have to remember to hit the save button.
I just did a new test category, named it "Current Arranged Trades', set it to private. Then I went through my plant inventory list, clicked on the edit button and clicked to add it to that new category and clicked 'Save your changes'. It would be the same to add a new plant from the database, click the 'add this plant to my list' and select the correct category. If it's set as a private category, only you would see it.
