Not really an instruction you're looking for, but
this post may give you a general idea except for updating CRES.
About updating CRES: Pets updated CRES of tubs to add dog washing slots. As CRES includes custom object's unique Modelname, I cannot provide a canned package which can be applied universally.
To update CRES, you must make a fresh clone of custom object
with the exact same Modelname of your custom tub. If something in the Filename is different from the original tub, you're doing it wrong. Make sure to pick the correct tub that your custom tub is based on, otherwise object footprint will not match.
Extract updated CRES from your fresh clone. Open the original custom tub and replace. Make sure to keep the original TGI intact.