I'm Sicness, a on/off support player, who normally swaps between jg/support. Whenever I swap to playing support Xerath and brand are my go to picks. I've been plat for around a season now, and am currently going for diamond.
This guide is what works for me and what I have found success with. I will be updating it whenever I feel like doing it, GLHF in your games :D

It is very important to think about how your opponents will win, as that will dictate how you play this phase. If your opponents are very aggressive(leona and lucian for example,) you might want to play further back than a lane like jinx yuumi, who would rather farm it out. Into aggressive lanes you want to start Q and play behind your wave, try not to hit the minions. Into a more passive lane you might want to walk into the middle bush and hit q's from there. You can walk out of bush, auto the enemy for pasive then walk back in quickly and avoid all minion damage.

Try to poke as much as possible, around this time you might want to start using your ultimate to roam. Your tempest + horizon powerspike is large, and if your able to constantly hit most of your q's and w's in a teamfight, the damage will be clearly visible.

In my opinion Xerath feels strongest late game, try to save your e for self or adc peel, and r for teamfights. Your ultimate can either be used as an engage to try to kill a squishy, when you run out of abilties or as a finisher.