Make relaxing fluid.
I take Lysol concentrate and add some to water.
Putting it in a sealed Tupperware with paper towels wet with the relaxing fluid softens them up perfectly the amount of days depends on the size of the beetle.
I like pinning them with the legs out, I prefer pins because it is tried and true and makes inspection of the underside easy.
If it drops the price of my big specimens! I don't care, beetles with the legs all folded up under and not pinned don't make any sense to me, unless you are just saving it to sell one day.