Redlining is when you draw over the artists' work with corrections. You should include notes about why you changed something to help the artist along. Here are some examples I found on http://cgi.4chan.org/ic/imgboard.html
Original: Redline:
Another Example:
If you don't have a tablet, print the image and use a different colored medium to make the corrections. If your corrections are the same color as the artwork, it will be difficult to see what you've done.