1.

Settings > Inventory > Checked Packages & Sales > Checked Product Packagings

2. 

Inventory > Configuration: Product Packagings

3. 

Create new Product Packagings > Fill package type, contained qty, qty per unit pack 

4. 

You can setup Package type for details
For ex: Pallet

5. 

Create new SO > Fill the product which had been set > Validate qty > Confirm 

6. 

Actions button > Do Custom Declaration > Pop-up screen appears > Confirm 

Note: you can check that which SO related to this customs and  in sales order also can see the related CD

7. 

Go to tab Declaration Details > Adjust some fields you want

For ex: Update Qty: 2

Declared qty: 2

NW:1

GW:2

CBM: 1.5

Note:  NW = Net Weight, GW = Gross Weight, CBM = Cubic Meter

8.

Confirm > Complete Custom Declaration > Shipped > Done 

9. 

Go back to SO > you can check that Fal Sum Qty Declared: 2 so remaining qty: 1 also Packaging field show your package