Once you have your custom furni made, add it into your hof furni, then insert it into the catalogue.
Follow the procedure that all the other furni you have.
A small piece of advice i found helpful when adding in customs,
Your adding a custom throne, extract or write down the structure of the throne (all the fields) and then insert a new row,
copy all of the other (the one you have taken data from) insert that into the new row and edit the ID, Name, Description, CCT name to match the new thrones data