I'm playing karthus for over 6 years
I have over 2M maestry points on karhtus
Peaked grandmaster 603LP[*] with over 65%wr playing only karthus
acc I'm curently playing on
https://www.leagueofgraphs.com/summoner/euw/SUPER%20MEGA%20ROLLO-ZAHIR
I sometimes streams on twitch
https://www.twitch.tv/kociokwik_

Karthus bot is really strong this sezon with new quest but remember botlane quest is strong but not game changeing don't force plates to finish it faser. Just lane normally and play for tempo you will finish it anyway.

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 want to all in or when you want to recall. Also you can put deep vision in enemy jungle if you crashed wave.

After you take down bot turret or enemy swaps go mid (if your mid is stronger and can you can match enemy mid it's fine to stay bot and let your mid progress madp). Play with team look all the time what they are doing. Karthus has big aoe dmg so gruping is better for him. Try to catch some free waves on side lanes if your teammates don't have tp, you can always ult if they fight. Sometimes it's better to split and fix side wayws bcs of your ult.(there is no rule for that, every game is different and you have to decide what to do).

Karthus can't engage on his own so you have to wait for your team to engage try to avoid getting poked to 0. Sometimes it's better to ult beofre objective so enemy team can't contest it (ofc teamcomp depentent). Karthus ult is very valuable in late game so avoid fighting without it. Look on objectives timer and your ult cd sometimes it's better to keep it for important teamfight rather than getting 1 kill.

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 as many minions as you can with aa, playing on practice tool might help you with that
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
9.5 Sometimess it's better to keep useing Q's during passive than ulting
10 Don't be affreid to flash on enemy team during teamfights you can deal massive dmg durring your passive
11 Track enemy jungler so you know when to push
12 Play for your limits (take fights that you are sure you win and don't over force)
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 phase to slow enemys so your support can engage on them, in late game don't be affreid to use it 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
18 IF you are new to karthus i recomend watching some karthus content on yt sadly there isn't muche but i would recomnd Karhus-NA streamer and old EloSanta videos to Lern karthus tech
19 Try different runes/items to find your playstyle
20 If you are gonna play kartus bot show him in lobby so your team can adapt draft to that
21 Ban your most hated matchup for me it's Jinx
22 If you want to climb faster find duo bcs playing with naut/pyke makes karthus twice as strong
23 Ping your support what you want to do
24 Karthus has many mid mechaps but only few that are tragic so you can pick him high in draft
25 You don't have to carry all the games somtetimes it's better to let your team carry and be weakside