There are several methods, but You can simply homogenize the cells in a Potter, in a buffer, then You can centrifugate, discard the pellleted material and recover the superrnatant.

Ok, If You need to control the proPO activation, it is better that You select correctly a buffer able to inhibit the activation, maybe an anticoagulant buffer and work at low temp (4-5 degrees). Finally if you do not need to reactivate the proPO, you can add to the buffer some Phenyltiourea (PTU) that blocks the phenoloxidase activation.