One good idea is to first run your own house ads which you control. That way, you can see how the placements look and move them around, etc. because you are using your own “house” images for display ads to get everything working as you expect.
Then, when you are “all set”, and you are happy with your ad layout and that your house ads are running as expected, switch over to your network ad tags.
This is the standard way to go in the beginning. Test and get your own house ads (images) working as you expect, and they switch over to network tags.