For doing ghusl e janabat, I have heard that first , the right part of whole body should be washed from up to down and then the left part of the body from up to down. My question is that is I dont follow this manner and wash my whole body from up to down together , it will be valid? I mean if I dont pay much attention on washing right first and then left, but I first wash head , then neck, then body and all?

You must follow the order first head and neck then right part followed by left