The process is super simple and the same whether creating for mobile or desktop, in-app or web.
Below are some screenshots for creating an ad unit. There is also additional information within Google Support Docs
1. Inventory > Ad Unit > New ad unit
2. Name New Ad Unit (replace any spaces with underscores _ ) and set any other optional settings (Sizes, Fcaps, Refresh rate, etc), then Save Ad Unit
Then you can repeat this for all your additional line items needed. Don't hesitate to reach out if you have any questions!