Find a reference image preferably with a scale in it.
Next find out what scale your drawing is in. (Feet, Meter, etc.) It should show on the scale.
This scale is in feet, as noted by the 5'. Others may say 5mm for millimeters, or 5m for meters.
Next before importing your drawing into Autocad, youll want to make sure the units you are using in AutoCAD are correct. Use the command UNITS or UN for short to bring up the drawing units tab which will allow you to specify the drawing units. For architectural we can set the Length > Type to Architectural instead of decimal.
Next you'll want to import your drawing into AutoCAD using the XREF (external reference) command. Keep in mind that XREF uses only certain types of images. For the purposes of this tutorial, we will be importing using a JPEG image.
When imputing your image it will as you for many input options, for now just get the image on to the model space.
Next use the command SCALE. This will allow you to scale your drawings to a certain dimension. Once you use scale, it will ask you to pick your objects. Select the image and press enter. Then it will ask you for your base point, which is the fixed location the image will be scaling from. Pick the point on the drawing scale labeled 0'. Then it will ask you for the scale factor in the input bar ( the bar on the bottom of the screen showing all the commands enter). You'll want to press r then hit enter to choose a reference instead of imputing a scale factor.
Drag the mouse from your base point to the end of the reference length you want and then input its new value.
When entering the new value be sure to input the number correctly.
For example if the length is 1 foot, don't just type in 1, but instead type in 1'.
Voila! Your drawing is scaled.
If your drawing is in Meters, you'll want to use an online conversion calculator to find out what value to put once you have your reference length. (ex. 1 meter = 3.28084 feet.)
Use the drawing scale on the bottom as your reference.
You can use the command DI, or distance to check the dimensions of the drawing afterwards.