AdButler has a range of helpful documentation that can be found here, https://www.adbutler.com/help/guides
Create your AdButler account using this link www.adbutler.com.
Note: you may create a trial account, but this will limit your functionality.
This video gives a quick overview of some basic training of AdButler, https://www.youtube.com/watch?v=uKIiKVbZXkA
Set up your server breakdown. You can add content/ads as you want.
Follow the AdButler guide to generate a VAST ad item/link
Note: Video files uploaded to AdButler cannot exceed 50MB