Here are a few tips and tricks that should help if you want to start developing custom activities within the new Sitecore Marketing automation engine.

The docs

There is a pretty extensive guide on the Sitecore docs site: https://doc.sitecore.net/developers/xp/marketing-automation/activities/activity-types/create-an-activity-type.html

Alternatively have a look at these really useful 4 blog posts: https://www.brimit.com/blog/sitecore-9-custom-marketing-automation-action

Debugging your code

There are a few ways to run the engine – by default it gets installed as a windows service however if you navigate to:

{xconnectdeployment}\App_data\jobs\continuous\AutomationEngine there is an exe (maengine.exe) you can run instead. Your code deployment will involve copying dll’s from your solution into that same folder.

To debug, run the console app and then attach to the maengine process within visual studio. When the plans run, and your activity is triggered, you should see breakpoints kick in. 

If you want to make attaching to the engine simpler, in Visual Studio ‘Add existing project‘ => Select the maengine.exe on disk => Right click ‘Debug => Start new instance‘

Some developer tips

If you are getting started and can’t seem to get your code to run, try setting up a very simple plan with a loose trigger. An example trigger, where the month is October. 

Also, check the custom config is in place to link your custom code with the GUID of the new activity within Sitecore. This needs to live in 
{xconnectdeployment}\App_data\jobs\continuous \AutomationEngine\App_Data\Config\sitecore\PatchFolder  – you can call the PatchFolder what you need.

And finally, if you want to see what Sitecore is storing under the hood when you save a plan, they live in /sitecore/system/Marketing Control Panel/Automation Plans. You can always un-bucket the folder to see each plan.