In this exercise, we'll be looking at how to add fields via a lookup field, with an example of the most commonly requested report type - Project Activity with Predecessor Activity.
Navigate to Setup > Create > Report Types > select New Custom Report Type button.
Create a report type with only Project Activities (or any other object) selected. Once saved, select the Edit Layout button:
From there, select the Create New Section button and name it Predecessor Activity (or whichever lookup field you're wanting to add). Once saved, select the lookup field to add your Predecessor Activity fields:
Select the Predecessor Activity lookup option and select all:
Once those fields are added, click and drag the fields (they'll have a magnifying glass next to them) and place them in the section you created:
Save your report type!