There is no proof that 'old masters' used maroger. The discussion has occupied both proponents and opponents for quite a long time now. At one point, I was pro-maroger. However, there is quite a lot of conservator information, and I no longer use it, and no longer advocate using it.
See this thread: Maroger Medium
. Read all 11 pages
, paying special attention to the posts of Virgil Elliott.
If you still want to try maroger, get it from someone like Studio Products or Old Masters Maroger.
Making the stuff yourself is somewhat complicated. First, you have to make a double-mastic varnish, a dangerous process of dissolving mastic tears in heated turpentine. Then, you must make black oil by combining a drying oil with litharge or lead carbonate, or a combination, and heating it. The oil must rest for at least 3 months so that the lead solids can settle. THEN you combine equal parts of mastic varnish and black oil to make the maroger.
I hope this helps.