My best idea is to freeze the body, that way it won't be so flexible and messy when you do the next step. Next chop up the body into smaller pieces and break the bones thoroughly. Make sure it is enough to fit into a bag without being suspicious. Lastly go somewhere with deep water, alone and quietly! and dispose of the small remains here. And remember to re-freeze the pieces constantly so no blood is shown. Another alternative could be burning, but you could run the risk of somebody smelling it. But it is also acceptaple if you are absolutely sure nobody will walk in/ get suspicious.