Open the Plot Dialog box
Pick your Plotter/Printer
Pick your Paper size
Uncheck Fit to Paper
Make sure the scale is 1:1 so 1 mm = 1 unit
It may also be helpful to use "Window" under "What to Plot" to center your objects on the paper
Make sure to use the preview to verify before you plot.
Having said that, if you're going to plot from AutoCAD with any frequency, I would create a paperspace layout for your plotter/paper, and plot from there, rather than directly from modelspace, as you'll be able to preview your plot at scale as it will appear on the paper before even entering the plot dialog.