The Sample Browser project includes a Getting Started series that walks through the full process of building a syntax language. It also shows some tradeoffs and options along the way, such as using a dynamic or programmatic lexer.
A parser grammar has to be written by hand but it is helpful to have the Language Designer code generate the AST node classes, if you plan on using type-specific AST nodes (recommended). Then you just write your grammar productions's tree construction code to construct the AST using those AST nodes you generated.
The Language Designer can also debug your grammar, so if you run into issues while writing it, you can run the debugger on it with some test input and see where things are going wrong.
For documentation, it is easiest to run through the Table of Contents. The LL(*) Parser Generator area within the SyntaxEditor area has info related to the grammar design.