Get rid of any dust, dirt or algae with a stiff brush. Pack any deep gaps, if required.
Cut the nozzle at a 45 degree angle to a width matching the gap you're going to seal. Fit the cartridge into a standard sealant gun. Apply slowly and evenly at an angle to the repair, making sure it covers both sides of the repair. If you're joining gutters, apply sealant to both surfaces and press them together. Smooth down with a wetted spatula. The product may remain tacky after a few hours of application, so don't touch it.
Clean all your equipment with white spirit.