Hi dear community!
First of all, my name is Christoph, 42 years old and i just started a job as SharePoint Consultant. I´m a career changer, formaly technical editor, and i´m an absolute beginner when it comes to SharePoint and Nintex. 

I need some small stepped help with a workflow i have to create for my company.
I want to create a workflow that daily checks a calendar for items that will begin in 7 days and sends once a notification to the assigned User.

I found following thread: 
Workflow to check calender for Items in future, to generate folder & tasks 

I started creating a Site Workflow, similar to the example above.
• Calculate Date: -7 days from today and write it to variable VarBerDatum -> works

• Lookup in List: Calendar1 Filter: Start time is smaller/equal VarBerDatum, write element IDs in collection SamElementIDs -> Don´t know if it works

• For each: This is where my problems start. I dont get how to configure it. 

How do i move on?
I know that i have to also set a metadata for the elements, from which the assigned user already obtained a notification. But on Site Workflow level there is not "Set Field Value" Action.

Please be gentle, i´m an old man

Thanks in advance!