A quick look at the QGIS Georeferencer that helps you place scanned maps or downloaded pictures, correctly on a map.

1. Try and select a map projection that is close to the scanned map original. Use that in your QGIS project and in the georeferencer.
2. Start with at least two points, so that you can select "linear" transformation and activate the link between the georeferencer and QGIS.
3. If you have high residual numbers for your additional points, you need to increase the transformation complexity. You need to balance this with sufficient number of reference points.
4. Try the georeferensing in QGIS, set some transparency to check it. Repeat and edit the points and transformation settings as needed.

Pro Tip:
If your scanned map is rotated, you can rotate the QGIS Canvas to match when you are using the georeferencer.