How to create a custom .dat file and write custom NBT Tag Compounds to it

[Mew]

I am wondering how I would go about this. This seems the best option to me as I can't seem to get anything else to really work... A tutorial on IExtendedEntityProperties or using the Entity.getEntityData() method. But as nothing seems to be avaliable with these, I want to create my own custom .dat file or whatever file is needed for NBTs'.

[MrObsidy24]

The link you sent 404s on my machine