To properly flash where your siding meets the concrete, you must use self-adhesive flashing tape designed for sealing around the edges. Start by cleaning the area where the siding meets the concrete with a cloth and a solution of mild detergent and water. Allow the surface to dry. Cut a piece of the flashing tape slightly larger than the area it will cover and press it firmly into place. Use a putty knife to smooth out any air bubbles or wrinkles.