You have to saponify it with base and then decarboxylate the acid in acidic conditions.
Suggested procedure for 200 g:
Dissolve 45 g NaOh in 1 liter or more of water andwarm this up to 60 to 70 °C. Then whilst stirring like mad slowly add the BMK as powder. (If it is no powder but lumpy yank it through a coffee grinder). The reaction is instant under those conditions. Stir for 10 more minutes at 70 °C. Everything should be in solution now, if there is anything not dissolved add some more water, if it is still not dissolved it is crap you do not want. Filter the solution (vacuum filtration with Celite pad is best but a coffee filter works too, you have to change it when it clogs up).
Let cool down to rt (room temperature) or less and add whilst stirring 120 ml of 37% HCl (thats 45 g HCl, 37% HCl has 37 g HCl per 100 g, 30% HCl has 30 g HCl per 100 g - note it is by gram not by volume!). Heat until it starts foaming, thats the CO2 released by the decarboxylation, when the foaming subsides stirr at the reached temperature for 10 more minutes. Stop heating and add 200 g of table salt (coarse, dishwasher regeneration salt is best) and stir until this has dissolved (or very most of it at least). It has now also cooled down a good bit and now you add sodium bicarbonate (baking powder) what will cause a lot more foaming. Do this until upon addition there is not much bubbles forming anymore. Warning! Stir and wait between additions, the reaction can be delayed and if you add it too fast you have it foaming over for sure!
P2P will be on top. Separate and wash twice with water and it will be ready to go.
Do not skip the filtration and the water washes!
When the final reaction is cold P2P separates nicely from the salt-water layer and no solvent is needed for extraction. If you want to use a solvent add it before the bicarbonate. Suited solvents are: Toluene, Xylene, Ethylacetate/Naphta 1/1. DCM is no good here.
Yield will be (depending on the quality of the BMK) about 120g P2P from 200 g BMK.