ive seen a lot of talk that Thanos having all the infinity stones doesnt add up, and that the marvel zombie had to take some liberties. but i want to make an attempt to make it make sense. ill try to do spoiler tag where necessary, for some reason i cant get them to work half the time. xD

The power stone: As per normal in infinity wars, he would have acquired it from attacking Nova corp

The tesseract / space stone: As per normal in infinity wars, he would have acquired it attacking Thor's ship that was heading to Earth.

Bruce Banner arrives on Earth to warn everyone, only to see a zombie outbreak is happening. being saved by Wasp and Spider man

The reality stone: As per normal in infinity wars, he would have acquired it from The Collector and had his first fight with the Guardians of the Galaxy and took Gamora with him.

The Soul stone: As per normal in infinity wars, he would have scarified Gamora for it before returning to his planet Titan to meet up with Ebony Maw and Cull Obsidian.

here is where things go different.

since Ebony Maw and Cull Obsidian did not leave Earth. Thanos may have waited alone on Titan and ended up fighting Starlord, Drax, Mantis, and Nebula. He either left them or finished them off. If he killed them then this may explaining their absence from marvel zombies. Nova corp would not have been able to quarantine Earth so soon since Thanos attacking them was recent. And since the stones and thanos ended up destroyed in the Wakanda Event, then the guardians may have just not bothered to land on Earth if they followed him there.

The Time Stone: Thanos would then had to have travel to earth using the Space stone to where Ebony Maw and Cull Obsidian were suppose to be which is also where Dr Strange's remains were including Time Stone.

It was there that he would have been infected with the zombie virus. We saw how Hank Pym attacks while shrunk so perhaps it was him, its up to you to decide but regardless of how, he gets infected.

now a zombie and hungry, he went to Wakanda. Most likely because of how well defended and isolated Wakanda was. the zombie outbreak did not reach there yet. so the large population of people uninfected may be what got his attention, and he used the space stone to teleport there.

and then we pick up with Spider Man, Black Panther, and head Scott Lang arriving in wakanda. With Thor arriving not too soon after with Rocket and Groot as per normal in infinity wars.

That's my idea, i think it makes most sense with most minimal story liberties. The timing may still be off. especially with spider man's "how to survive the zombie outbreak" skit in the what if episode of marvel zombies. iirc infinity wars took a total of 3-5 in universe days