Practice, practice, practice.
On the weekend, when you have plenty of time to do it, leave the dog alone and in her crate. Leave a toy for her, and give her a treat. Then step outside your door and close the door. Immediately open it again and step back in. When you come back inside, let her out of her crate and praise her a whole bunch.
About an hour later, do it again, but this time, stay out for the count of five. Then later, do it again, and stay out for about 10 seconds. Keep increasing the amount of time you're outside and she's inside in the crate.
If she starts barking or whining before the time is up, go back in and tell her "No." Don't scold her or yell at her or anything like that, just tell her calmly, but with authority, "No" or "Stop" or whatever word you want to use. Then go back out again for the prescribed amount of time.
It may take more than one weekend, but if you're consistent and persistent about it, she'll soon learn that you really will come back.
Good luck!