Xerath in the early game is one of the most micro-heavy champs I think I have ever played, especially as support. Having to deal with poking for money with Spellthief's Edge, and keeping track of Mana Surge to make sure you always have some mana is the most bothersome part of his kit, up until you get Lost Chapter. In the early game as a support, keep poking with your Arcanopulse to try to pump out money and damage. This also helps with getting Dark Harvest stacks, which are very useful in the later game. By far the most important thing to keep track of with Xerath would be his passive before you have Lost Chapter, as he runs out of mana very fast then he's practically useless in lane. Just make sure to keep the pokes going as often as you can, and always auto attack a minion every 12 seconds (or a champion if you are able to without being damaged).

After you buy Luden's Tempest, you are generally going to want to stay in lane and get a Tear of the Goddess as soon as possible. Getting this allows for you to get stacks, and sometimes I'll buy this before Luden's Tempest if my ADC is playing more aggressively. Buying Tear of the Goddess at this point allows you to finish Horizon Focus, then move on to Seraph's Embrace.

In the midgame, Xerath begins to actually do things. He is able to roam to mid a bit now, as he would be able to help assist the midlaner in their fight. So keep in mind to roam mid in midgame, and being able to do this will also get you those nice Dark Harvest stacks. Once you get the first 3 items, you should be able to get close to one-shotting people, and you definitely will be able to one shot the squishes once you get Rabadon's Deathcap. The most important part of the midgame is using Rite of the Arcane to get Dark Harvest stacks on any person under 50% health, assuming you're fine with getting yelled at in chat about stealing kills. Make sure to keep track of objectives, and to use your Arcanopulse and your Eye of Destruction to kill the epic jungle monsters, and to always save your Shocking Orb for any fight that might come during or after the attempt to take the epic jungle monster. Map awareness is very important on Xerath, and being able to use your Rite of the Arcane to clean up or for Dark Harvest stacks is very important.

Once you have reached the capability to one-shot, you will be targeted first in teamfights. To a Xerath, poking is the entire game, from laning phase to end game. As you are an artillery mage with every ability being a skillshot, you can't really afford to miss in the late game. Especially with how squishy you are and how vulnerable you are after you use your Shocking Orb. Being able to one shot almost any non-tank is an invaluable part of any teamfight, but chances are you won't be able to use it. Making sure to stay back and use Arcanopulse and Shocking Orb will be super important. Only really use Eye of Destruction when you stun someone and you won't be able to be traded. If you get traded 1 for 1 as Xerath, that's a negative trade for you as it prevents your high damage output and ability to clean up.

Hope you have fun as Xerath!