You will need a eBay selling manager or selling manager pro account.
1. Navigate to your eBay selling account.
2. Click on 'Active' on the left hand side navigation.
3. Under the Actions column dropdown select 'Revise'
4. From the Edit Your Listing page, click Add/Edit Variations which is under the 'listing multiple variations of your item'
5. Click Continue through to the third page, titled Review Your Variations. Check Add a SKU for your item. In the Custom Label (SKU) field, enter a base SKU, then click Add.
6. The base SKU is now at the beginning of each variation SKU. Edit the variation SKUs to match your other channels, then click Continue.
7. Next you can navigate back to ecomdash and download your listings for eBay to import the variation SKUs.