1st stage: Decompile the selected SWF you would like to customize using Decompiling software, such as Sothink. Then, design each images of the SWF using image editting software such as Paint.net or Photoshop. Then edit the images inside the SWF and recompile the SWF
2nd stage: You need to hex edit the SWF and replace the names (im not really sure how to hex edit)
3rd stage: Create an SQL for your DB, the SQL should include the following:
Catalogue_items
Furniture
Once this is finished, you would also need to add a line to your furnidata.txt (and possibly productdata.txt depending on your furni) Then run the commands in game; update_items ; update_catalogue ; then refresh your cache to see the custom.
Or you can just take an old custom and redesign it using step 1 without having to worry about hex editting or creating the SQL