From MK to WL there is a direct boat from the MK Marina which is right outside the MK gate, so take the boat to WL. Getting back to CSR requires, if using Disney busses for you to go to a park or Downtown Disney first and switch busses at one of those. Depending on time you leave WL, you may be able to return by the same boat to MK -- if MK closes at 8, that really means it does not close until close to 9 (rides and attractions stop at 8, Main Street stores remain open for usually another hour. Thus boats to MK will still be running till then and bus from MK to CSR will still be running till then. Second option is to take bus from WL to Epcot and switch there to bus to CSR -- Epcot remains open to 9 officially but that is when Illuminations starts and thus is really open to almost 10 before the masses depart. Other two but slower options are to take bus from WL to MGM (if still open), or to Downtown Disney, and switch at either of those.
Fastest way for about $12 is to take a cab from WL which are available through valet services which are right outside the main entrance door to WL. Cab trip from WL to CSR takes 15 minutes or less. Those trips back mentioned above using Disney transportation can often take 1 to 1 1/2 hours combined traveling and waiting time.