This isn't a complicated achievement, but it can be tedious if you are only going for this one.
You will need The Old Gods DLC, and maybe the Rajas of India DLC to do it a little bit faster.
First of all, you need to start as a norse character in the 867 start, personally I recommend using Halfdan Whiteshirt of Jorvik because you already start in England, and has a lot of event troops.
Then, you have to conquer a province with French/Occitan/Breton culture before 1150 so you can become Norman culture. (I went with Devon and Cornwall, on Wales)
You will need to wait until an event fires that says "Province converted to Norman culture" or something like that, and then you move your capital to that province, so you can switch the culture of your ruler. If you have the Rajas of India DLC, you can use a decision (it costs 500 prestige) to switch the culture. If not, you need to wait to another random event to fire that allows you to switch
After becoming Norman, you will have to switch to English culture (the same process), you will need to rule Anglo-Saxon provinces in the England region after 1100 , so that involves a LOT of waiting.
So, let's focus on the other part of the achievement. As a pagan religion, you have CB's that allow you to conquer England easily, so i recommend switching to Christian after conquering England, but you can do it before if you want.
It's important that you stay Norman until 1100, so keep an eye on your ruler and the heir, and I recommend doing other achievements in the 200+ years of waiting you will have ;)