Geo-referencing is the process of taking a digital image and adding geographic information to it so that GIS software can 'place' the image in its appropriate real world location. Let's take for example the same Survey of India map we used in module 1E - sheet 52d/6. First step is to convert the PDF format to a regular JPG file through any website, e.g. ilovepdf.com. See image 1 below

Next, in QGIS select "Georeferencer" from the "Raster" menu and click Open raster (blue tile) icon to load the JPG file. Next click on each of the 4 corners of the map (excluding the boundary) and enter the corresponding latitude and longitude mentioned on the map. Important: enter the lat / lon in decimal degrees (e.g. 76 deg 15 min corresponds to 76.25 decimal degrees as there are 60 minutes in one degree). Ensure to set the CRS as "EPSG:4326 - WGS 84".  See image 2 below.

Once done for all 4 corners, click the georeferencer icon (green triangle) and set the default Transformation type and Target SRS. Click OK. You ll have to click the georeferencer icon once again to do the actual georeferencing. After this a new raster layer will be added to QGIS showing the Survey map in its appropriate location on the basemap. See Image 3+4 below

You can zoom in and hide/show the new SOI map layer to verify the matching location of streams, settlements, roads, etc. on the OSM basemap.