If it is a large area to fix a patch or a piece of inner tube cut to size will fix it nicely. Use the shoe goo and it will last longer than the rest of the boot. Clean the rubber first and use a pencil to mark out the area the patch will set. Then smear the glue to the line. This way it won't look bad with too much squeeze out. You will get better at fixing as you go. Practice makes perfect. I use the shoe goo to fix more than boots. It has very few limitations.