Simplistic Notation example

Notation package looks poorly documented.

For example, GridBoxDividers option is not documented at all.

Can someone give me a simplistic example of a Notation set up? For example, suppose I wish to represent

MyHead[a,b]

as

$\frac{a}{b}$

with

GridBox[{{"a"}, {"b"}}, GridBoxDividers -> {"Rows" -> {{False, True}}}]

If I enter

Notation[GridBox[{{ToString[x_]}, {ToString[y_]}}, 
   GridBoxDividers -> {"Rows" -> {{False, True}}}]
\[DoubleLongLeftRightArrow]
MyHead[x_, y_]]   

I get an error

Notation::noexbtag: The external representation GridBox[{{x_},{y_}},GridBoxDividers->{Rows->{{False,True}}}] does not have an embedded TemplateBox with tag NotationTemplateTag. The Notation statement Notation[GridBox[{{x_},{y_}},GridBoxDividers->{Rows->{{<<2>>}}}][DoubleLongLeftRightArrow]MyHead[x_,y_]] may not have been entered using the palette, or the embedded TemplateBox may have been deleted. The embedded TemplateBox ensures correct parsing and retention of proper styling and grouping information. >>