Iím afraid what you have isnít enough to make an essential oil - these are made by steam distillation - that means boiling the material in water, condensing the steam and then separating out the oil from the water. Not impossible to do with home made equipment but quite difficult. If you have the DIY skills you could modify a pressure cooker to do it - I think someone here has done just that so may advise.
However there is another problem: Iíve never actually seen an essential oil of tonka. Tonka beans are normally used as an absolute, which is solvent extracted (with hexane in this case I think), that produces what is called a concrete, which in turn is washed in ethanol to produce the absolute. In theory that too could be done at home, but hexane is not nice stuff to work with and I would be very concerned about safety.
Instead can I suggest you make a tincture? Grind up the beans, weigh what you have and put it into about three times the weight of perfumers alcohol in a container you can seal. Shake regularly and check every week or so for strength of odour in the ethanol. After a month or two youíll probably have a usable material, which may well need dilution before adding to perfumes.
I should add that Iíve never made a tincture of tonka beans - so if someone has tried this please comment in case there is something Iíve overlooked that means it wonít work!