Prove: Every decidable set is Turing reducible to the empty set
You're trying to prove something, so you need to use something closer to an actual definition of Turing reducibility. The statement you quote from Sipser is an intuition about what reductions do, but it's not precise enough to let you solve problems.

Here's an informal definition of Turing reducibility that lets you start to reason about what it can do.


  A set&nbsp;$A$ is Turing-reducible to a set&nbsp;$B$ if, assuming the existence of a subroutine that determines whether its input is in&nbsp;$B$, we can write a program that determines whether its input is in&nbsp;$A$.


We're considering $B=\emptyset$, so we need to assume the existence of a subroutine that determines whether its input is in&nbsp;$\emptyset$. Well, that's not much to assume. Here's the subroutine:

 return false


OK&nbsp;&ndash; we don't need to assume anything: we know this subroutine exists.  So now, we just need to show that every decidable set is Turing reducible to&nbsp;$\emptyset$. That is, we need to show that


  For every decidable set&nbsp;$A$, we can write a program that determines whether its input is in&nbsp;$A$.


So... what's the definition of decidable?