Re-topology workflow in Blender is pretty straight forward and convenient,
Even without using a dedicated addon like ‘Bsurfaces’.
The simplest way to create new geometry over an existing surface is to activate snapping in face mode, and activate the ‘Project individual elements on the surface of other objects’ option.
This let’s you extend geometry, create new elements and transform existing elements while they’re constantly sticking to the surface behind them.
This can also be used with ‘Proportional Editing’ to allow organic tweaking of the new mesh over the surface.
Activating the ‘X Ray’ option in the display properties of the new low poly object helps by displaying the new mesh above the original surface.
Another helpful tool in the process is the ‘Shrink Wrap’ modifier,
It can be used “press” mesh parts on to the original surface and than applied to create surface-tight mesh that can serve as the basic mesh for Re-topology or be joined to an existing mesh that was created earlier in the Re-topology process.