In order to pack islands to a non-square texture, you need to enable the checkbox in the Non-Square Packing subpanel header:
Next perform the following steps:
- unwrap your islands in the usual manner so that they are not deformed when displayed with no texture open is the UV editor,
- open or create a non-square texture in the UV editor. You will notice that the islands will be deformed now,
- in the next step, adjust the islands to the texture, i.e. apply a non-uniform scaling so that they have correct proportions with the given texture. You can do it manually, you can also use the add-on to perform scaling automatically. In order to do that, select all islands you want to adjust and press the Adjust Islands To Texture button in the Non-Square Packing subpanel. You will see that islands will have correct proportions after the operation is done (they will no longer be deforemed). Note that the adjusting operation must be performed only once for the given set of islands
- from now on you can run the packing operation normally. You can pack islands multiple times and their proportions will always be correct with the given non-square texture. You only have to make sure that you have the Non-Square Packing functionality enabled all the time when you want to pack islands to the given non-square texture.
You can also undo the adjustment operation. In order to do that, select the given islands and press the Undo Islands Adjustment button in the Non-Square Packing subpanel. After the operation is done, the islands will be deformed again when displayed with the given non-square texture. When you close the texture, you will see that the islands will now have the correct proportions when displayed in the UV editor. Note that for the undo operation to work properly, the texture used at the time of undoing must have the same ratio as the texture the islands were originally adjusted to.