I use Selling Manager Pro so this may not relate to you.

On the left hand side of the Summary page click on Inventory.

This will move you to the current inventory.

At the bottom left - click on Create Product and follow the instructions.

I found the Inventory very hard to get my head around as I kept getting it confused with listing items for sale.

Once you have the inventory on file, you can select Sell Item (or something similar) and go through the listing procedure.  You then save any edits to the inventory so it remains on file even if all the items are sold.

Thanks very much I'll give it a go right now

OK that's straight up enough, what about moving current listings to inventory, is there a way you know I can do that please?

It's okay thanks I have found where I can save to inventory

You seem to have found your way around well.

One word of warning - the inventory stock control is not reliable.  When it lists the number of items active this is not necessarily the truth.