Go back through paragraph 1 looking for additional errors. You will almost certainly find one or two. If you find more than that, assume that you'll have work in multiple passes to identify all the errors.
Repeat steps 3 and 4 for the next paragraph.
Now that you're sure about how many passes through the text you'll need to spot all the errors, continue to mark all the errors you can find through at least two full pages of the text.
Work with the sample again until you feel comfortable with steps 1-6.