The data you have is already close to being a proper JSON file, but you need to remove all the "x item(s)" strings:
{4 items
"message":"accurate"
"cod":"200"
"count":1
"list":[1 item
0:{11 items
"id":2643743
"name":"London"
"coord":{2 items
"lat":51.5085
"lon":-0.1257
}
"main":{6 items
"temp":291.57
"feels_like":288.33
"temp_min":290.37
"temp_max":292.59
"pressure":1024
"humidity":42
}
"dt":1595275779
"wind":{2 items
"speed":3.1
"deg":10
}
"sys":{1 item
"country":"GB"
}
"rain":NULL
"snow":NULL
"clouds":{1 item
"all":7
}
"weather":[1 item
0:{4 items
"id":800
"main":"Clear"
"description":"clear sky"
"icon":"01n"
}
]
}
]
}
I have added indentations so you can see the different parts of the data. But since this is already in JSON format you just need to use a TextWriter to write it to the file. What you do with it after that is not clear.