So when you launch the program, it will automatically load your local costume file if it exists.
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"Add" button will add an empty slot
"Remove" button will delete the selected costume
Arrow up down will shift the order of the currently selected costume slot
The name of the slot won't change immediately so you know what went where.
On the top right, you will see "Edit" and "Transfer" tab. Select the "Transfer" tab and select "Open costume.sav to import from"
This will open a file select window. Navigate to the costume.sav file you've downloaded and open it through the window.
Now the box on the right should show all the costumes in the file. Select one of the slot to see the costume's preview. Press the "<<" button to import the selected costume to your save.
When you're done, select "File" then "Save" from the top left menu.