Create a Grid and the on the Grid Manager page select Flatten Grid next to the Grid. Then, select Edit.

I am pretty sure this is covered in the CMS documentation. Have you taken a look at it?

it is a post for oct 2011,sorry for post in it
but i can not find or i dont know where i must add code that Lynne said under
Layout Manger HTML:

When you flatten your grid and then click to edit it, you will add the code on that page.