It is a bit difficult to give an exact quantity of plant material, either by volume or by weight.  Much would depend on the individual plants, size of clippings, size of pot used, amount of water, etc.

As the instructions in the article and in the book state, you take a pot of any size and put clippings of oleander (leaves, very small twigs and flowers can all be used) and tamp them down without crushing and then add enough distilled water to where it barely covers the plant material.  Then you boil slowly according to instructions, strain and continue boiling, etc. until you have about 30% of the original volume of liquid, which you strain through several layers of unbleached coffee filters or else commercial filters and dilute 50/50 with vodka (or you could use distilled vinegar, though vodka is best).

You really do not have to be that precise, but if you want to measure more precisely, use a hygrometer and condense the liquid prior to dilution to the point that it measures 0.07 more in weight than an equal volume of water.