How do I configure the text capturing?

The rules that define how to capture text from an email are in the field settings. You need first to create a field and then select a capture method.

This task can be challenging and sometimes requires a lot of trial and error. The usual process is:

Create a new field under a Parser.
Select any of the captured methods. Starts after… continues until… is the most used one.
Double click on the email source and select one of the emails you want to capture the text from.
Go to the fields tab and copy the Body contents. Most of the times, the email body is the source of the data you want from the email.
Go back to the field you have just created and paste the text into the testing area.
Play with the options until you find that the testing area output is what you expect.

For more complex parsing tasks you can try other methods like Regular expressions, XPath for capturing text in an HTML document or even scripting.