completely disassemble the cube, including the screws springs and core. wash all the pieces in soap and water, rinse, and then let them dry (except the screws and springs, make sure to hang dry those). after everything is dry, reassemble and lube
if you're doing inspection for CFOP, and you see two cross pieces already in, but theyre in the wrong permutation (blue to the right of red on white cross, for example), would you put the other cross pieces in and then do M2 U2 M2, or would you replace one of the cross pieces to get the right permutation and then finish the cross?