Using Raster Bender plugin to rubbersheet raster layers in QGIS

I have a distorted raster of a historic map that I need to warp to match the modern map.

I’ve installed the plugin Raster Bender into QGIS 2.18, but I can’t work out how to use it, and can find no instructions online.

How do I set up the pairs of points for it to use to warp the raster?

If there is an alternative method for QGIS 3, that would do. I have both versions.