If you plan to install a flexi-harness on a fursuit base, there are several ways to do it. It's best to do this early in the process before adding ears, foam work or padding. Regardless of your chosen method, ensure the rectangle rings are still attached, and the elastic is not twisted. If you need to, you can always remove the harness (except for the rectangle rings) and re-thread it after installation.
Please remember to use appropriate safety equipment and work in a well-ventilated area or use a proper respirator, especially if you're working with paints, primers, glues, or other hazardous supplies.