Go to someplace like advance auto parts or autozone and buy some black windshield installation silicone...
follow the manufacturers instructions for using...basically if you can caulk a window or door on your home you can do it...
otherwise cut the tip about 1/4 inch back at a 45' angle...
keep the tip to the top and while holding the tube diagonally so that the cut stays in contact squeeze gently causing the material to flow out...like toothpaste.
continue on with a bead until the entire gap is filled...let sit for 24 hours and you should be good as new.
