1. Keep hitting "Save after every action or two.
2. If you mess up something EDIT - UNDO LAST is your best friend. (You can repeat as needed, or close your work and go back to your last SAVE.)
3. When you are ready to SELECT the color you want, really, REALLY enlarge the heck out of the image (the + on your keyboard). This gives you a much bigger area of color to SELECT for removal. When you think you have selected the correct color, check the bottom left corner to verify that you have indeed clicked on the correct color. If so, select your large word, hold SHIFT and select the small word. DIFFERENCE.
Hope this helps Friend. jerry