Skylight caulk
Skylights are installed on the roof because this is the only way you can have more natural light in the room during the entire day. Because they are installed on the roof, there is the problem of leaking, since you have to make a whole to install the skylight. In order to cover all the leaks you have to use caulk.
1. Get a ladder. Take you caulk gun loaded with polyurethane caulk and get up on the roof.
2. Start applying the caulk from the farthest corner from you. When you press the trigger try to keep it pressed gently to apply a smooth and even layer of caulk. Just advance along the seam between the skylight and the roof until you cover the entire perimeter with a layer of caulk.
3. Now it is time to seal the skylight from inside. This time load the gun with silicone caulk, but apply the same procedure.