Karthus has strong early so you can trade a lot and look for kills on lane. Karthus outscales most of botlane champions so doing trades 1 for 1 is worth also when are abaut to die do it in enemy wave so you will push it during passive. Also after 6 look what your teammates are doing bcs you have global ult. Ping your support when you wan to all in or when you want to recall. Also you can put deep vision in enemy jungle if you crashed wave.

After you finished bot turret go mid. Play with team look all the time waht they are doing. Try to catch some free waves on side lanes if your teammates don't have tp. Sometimes it's better to split bcs of your global ult.

karthus can't engage on his own so you have to wait for your team to enage try to avoid getting poked to 0. Sometimes it's better to ult beofre objectiv so enemy team can't contest it (ofc teamcomp depentent).

I'm karhtus otp since 2021
I have over 700K maestry points on him
Peaked master master 253LP with over 60%wr playing mainly karthus
my main acc:
https://www.leagueofgraphs.com/summoner/eune/Karthus%20DMG%20Hack-EUNE
my twitch where I sometimes stream in english/polish
https://www.twitch.tv/kociokwik_

1 Try to Q->aa->QQ->aa->Q when fighting
2 Kite with your Q
3 Use Q to poke when enemy adc is about to last hit minion and is locked in animation
4 Don't be afreid to die but you have to think if it will be worth it
5 In early game you can take execute under enemy tower to push 1 more wave do deny enemy minions and next wave will bounce back to you
6 Don't play on auto pilot play less game but keep focused on a game
7 Try to last hit minions with aa, sometimes it's better to crash a weave asap and miss few minions
8 Press tab often so you can see enemy items and decide if you are stronger and should fight also check for items like banshee or seaker's armguard (stop watch)
9 Try to hit some q then use ult in last secounds of passive
10 Don't be affreid to flash on enemy team during team fights
11 Track enemy jungler so you know when to push
12 Play for your limits
13 Use q and durimg animation use "w" to boost dmg and make sure you will hit it
14 You can use "q" to zone enemys and make them stay in your "e" and aa them to deah
15 Before you zhona use "q" and active item to hide animation
16 Use "w"during laneing fase to slow enemys so your support can engage on them or in late game so your team can catch someone
17 Some skins have more visible animations than oter i would recomend not useing Infernal Karthus and Lightsbane Karthus