Starting computer into Windows/ Returning to Mac side

Restart

the computer you intend to use
 

While

the screen still looks like this:

Hold option


key between control and command until the dual boot screen appears

If

you let it get to here:

the computer will boot into the MacOS and you will need to

start over

Once at the dual boot screen, click the hard drive icon with the operating system you want to use (presumably Windows)