The Main difference to most guides would be the runes. The whole idea is to outsustain the enemy using Fleet Footwork (FF) healing and your passive mana restoration in combination with Presence of Mind mana. The FF MS also lets you manouver much more easily, allowing for better chances to dodge skillshots or reposition your Q with the added MS. This allows for a very controlled early game, where farming for Lost Chapter or Hextech Alternator on first back is pretty simple. Even when ganked, there are many scenarios where you can still stay in lane with low HP, and don't lose XP/Farm thanks to the FF sustain. You will want to focus on last hitting until you hit Lvl 6, or you get a decent gank. When farming, try to use mostly AAs for the first wave, and then tanking the third wave in front of your turret to freeze it easily. this should also delay the enemy's Lvl 3, while you can last hit much more easily. Compared to later stages of the game, in the early game you want to keep most opponents at least roughly half of your Q range away from you. If you don't, you will probably die. The champions I mentioned as threats in lane are mostly assassins, and you mostly shouldn't play Xerath into assassins if you have a choice. Unless you want to assert dominance by picking Xerath into Talon. I respect that.

Once you hit Lvl 6, you should check if you can use your R bot or on Objectives (Obj). I will usually find pretty good angles in botlane, as long as the enemy lane has less than 70% HP there is a chance to secure an advantage for your team. Obj can be risky, because you can assume both your jungler and enemy jungler (provided your not in call with your jgl) have no idea what you're planning, or how much damage your R does. When going for botlane ganks, you should ping "On my way" roughly when your R is half a lane out of range. That way your bot can react in time to engage with your damage. You should aim to use your R as soon as it comes off cooldown (cd) in order to maximize your impact on the game. Xerath is very good at poking and controlling small areas from very far away, so you should use this, since everything else about Xerath is average at best. His burst is mostly not enough to one-shot without hitting multiple R shots (unless you are incredibly fed). Since this isn't one of his strengths I prefer to support my team from afar. It takes some time to figure out when NOT to R though, or rather where not to R. Basically you should avoid river and enemy jungle, the best spots when going for bot for example are the brushes in your jungle. That gives you just enough range to hit underneath enemy turret for cleanups. As soon as you hit Lvl 9 and gain access to Farsight Alteration (Blue ward), you should get it instead of Stealth Ward, since facechecking brushes is never a good idea as Xerath. Instead use W which reveals its AOE when it's damage goes through and your Q when you get Horizon Focus to check brushes.

Assuming neither the enemies nor your teammates have forfeited yet, we get to the part where Xerath is an actual champion. In regard to the playstyle I previously outlined, I like building either Horizon Focus or Malignance as my first item. Due to the sustain from your passive and PoM, you shouldn't run into any mana problems if you're able to AA at semi-regular intervals, even when opting for HF first. I feel like HF is very underrated right now, even though the AOE reveal is very powerful at dealing with flanks or to check objectives. This ties in perfectly with Xerath's kit and is in line with my playstyle. The Malignance burn zone is a nice added benefit, however the ultimate CDR from the item's passive is what you're really after. This will allow you to use your best ability much more often. Depending on how the early game went, you should try to fit in a Dark Seal to capitalize off of your Rs to the fullest extent. As a champ who likes to play at a very safe distance, DS and Mejai's are very valuably, especially when you reach 10 stacks on Mejai's due to the bonus MS. Another item I occasionally build is Cosmic Drive, due to its effect resembling Season 13 Luden's Tempest's effect, giving you a short burst of MS when hitting an ability. When hitting an enemy with your W or E, this gives you a huge difference in MS, allowing you to reposition to kill or escape. When you're fed enough, going for DCap is obviously good, however getting Zhanya's Hourglass is often my priority considering you want to keep your stacks (and your shutdown if you're fed). Building Rylai's in conjunction with Malignance is also viable, since this makes consecutive R shots landing more likely, allows your team to follow up more easily and is just very annoying to play against. Voidstaff or Cryptbloom are also great items on Xerath (every mage player hates MR). Personally I'm more fond of Voidstaff, but when opting for a supportive playstyle, Cryptbloom's heal can be very powerful in 5v4 scenarios (you're in base shooting random R shots, so its technically not a 5v5). Going for Archangel's Staff can be an option when against burst heavy champs, but the mana is somewhat wasted on you due to your already high sustain with this build (plus everyone has blue buff past minute 20 anyways)

Mostly, just be as much of a nuisance to everybody in range. It doesn't particularly matter if you get kills or not, you should still aim for assists in order to stack Mejai's. Try to go for immobile ADCs, enchanter supports, immobile Mages and champs that are low HP in that order. Getting the enemy ADC (or APC) to 30% is in most cases more important than getting kills. Unless you're a KDA warrior. Then be a kser. Being fed has a huge disadvantage, which is that you draw a lot of attention to yourself, especially if you have a fat shutdown on your head. That's why I usually buy the Mejai's upgrade after I died with 10 stacks instead of when I have 10 stacks (not always, but pretty often). This way, the enemies won't be as attracted by your big boy shutdown, while you're still as fed as ever, and scale even faster after getting Mejai's.