Always start bot lane with a leash. Get bot buff -> gromp -> and then second buff. Then you'll have around 20 seconds to look for a gank with level 3. You are able to invade as Sett because your level 3 is good. I usually see if I can catch them on their top buff with lower health than mine and either kill them or make the flash at the very least. Take both scuttles once they spawn, your E is able to remove the scuttle armor with ease.

Your goal as Sett with this build is to gank and have a strong early lead. You are healthier than most junglers and can take objectives with ease. Between Fleet, Celerity, and Waterwalking, your ability to rotate and stick to the enemy is massive. You should take full advantage of being able to keep up with most enemies.