To answer your question directly: You make a rule, then within the 'actions' of that rule you add an Adobe Analytics 'Set Variables' action. As you customize your 'set Variables' action, you get a nice UI in which you can set an evar or sprop directly to a data-element. You'll need to have rules that set variables, execute the proper beacon send, and then clear out the variables too.
One straightforward example is a typical pageload rule. When the page is done loading (let's say 'DOM Ready' in this case), you can set all the variables you want on that pageview within a set variables action -> send the pageview beacon -> clear the variables.
I would recommend checking out the Launch User Guide documentation for some general help on the differences of Launch's UI. The videos section (especially the first video) give a great walkthrough of the necessary things to understand in the platform. The first video is 20mins long, and I consider it a must-watch for anyone that is getting started with Launch, regardless of their other prior platform experience.
Link: https://docs.adobe.com/content/help/en/launch/using/intro/get-started/videos.html
