Draw in Model Space at full size. AutoCAD doesn't care if it's miles long
or inches long. You will actually be drawing in a default Viewport,
although you can't see any border to it.
In Paper Space, insert the border drawing you wish to use, then enter the
Viewport to scale what you have drawn. Scale the drawing to fit the
viewport. If you want someone to use a scale rule on the resulting drawing
then your scaling will need to be appropriate, but it is not recommended.
Those using a drawing should work to the dimensions given!
To learn about Model Space, Paper Space and Viewports you will have to do
some reading of the Help files and/or a book on AutoCAD.
Best of luck!