Recently I had the privilege and good fortune to present at the 2019 Sitecore Symposium in Orlando, Florida. Below are links to the slides and notes from my presentation on customising Sitecore Marketing Automation plans.
Github repository containing sample code, utilities and content package: https://github.com/parrya/ma-demo
Troubleshooting and debugging tips
See this blog post for some tips on creating custom activity types in Sitecore Marketing Automation plans.
Custom Activity Types in 9.0.x:
Automation deep dive:
Marketing Automation Achitecture: