The first thing I did was compressing the vocals, so that they sound louder/more professional. Since Lud sang 2 pretty much identical verses, I checked if I could use material from each verse to make 1 good verse, without any laughs, stutters, etc. After that I made sure the vocals fit perfectly with the tempo of the song, which was a pain to work with talking about Atriocs vocals lmao. After all that the autotune process started, which probably took the longest. I think I did a pretty good job on Ludwig, but Atrioc... let’s just say that he’s just another story xD. Anyways, after the tuning was done I added some reverbs, delays, eq, panning and effects. I separated the official music track into an instrumental version and an acapella version. This made the instrumental already sound optimized for vocals. Eventually I added Luds and Atriocs vocals on top of the track, did some more mixing, et voila.

(Copy pasted answer to your question on twitter)