Unfortunately that question is BIG and does not have a simple answer. Additionally, while there are some generalities that can apply, it is dependent on the nature of the company.
It is sort of like asking how do you build a building.
For example: Sometimes that building needs to have a swimming pool inside such as for a Recreational Facility and most other times not.
I'd recommend that you research the ERP systems that are on the web. Study all of the functional modules that they offer. Then you might get a general idea of all of the parts that you need to consider in your own ERP system.
Dynamics GP can, and does, offer a number of those functional modules, but depending on the specifics of the company involved, it may not provide all that you need.
Good Luck,
JRB-Bldr