After you have a new instance of D365FO running creating a new legal entity would be one of the next steps. The actual process of creating the legal entity is pretty easy. What you do with it beyond that point in terms of setting up the data to run the company is where the work is. One important point you should get right up-front when creating the legal entity is the region code. This turns on the local functionality related to that country or region if it is provided.
Here is a quick overview.
D365FO Update3 (7.0.4307.16141)
In this example the database is populated with other legal entities already and the initial check-list has been completed so there are some system parameters already created.
If you are looking for AX2012 example and additional details on the country context you can see this post.
As well it’s worth understanding the data structures in AX for example what is shared between legal entities and what is not. This post was done on AX2012 but the main concepts still apply in D365FO.