How would you separate a mixture of saltwater?
Depends what you mean by "a mixture of saltwater".......
If you mean "how do you separate saltwater into separate salt and separate water" the solution is simple.
You place the saltwater into a flask that is fitted with a water-cooled condenser. Heat the flask, and the saltwater eventually boils, giving off water vapour that you collect via the condenser. Continue the process until the flask contains just the dry residue, which is your salt.
However, if you actually meant "how do you separate a mixture of saltwater and.......[something else]" you need to say what the "something else" is!