After googling, I didn't find any tutorials but using JSONKit should be self explanatory.
After downloading your JSON feed using NSURLConnection or ASIHTTPRequest simply create a dictionary of all the objects in the JSON feed like so:
//jsonString is your downloaded string JSON Feed
NSDictionary *deserializedData = [jsonString objectFromJSONString];
//Helpful snippet to log all the deserialized objects and their keys
NSLog(@"%@", [deserializedData description]);
After creating a dictionary you can simply do something like this:
NSString *string = [deserializedData objectForKey:@"someJSONKey"];
And that is the basics behind JSONKit.
JSONKit is much more powerful of course, you can find some of the other things you can do with it in JSONKit.h