This is a very easy concept to teach a dog. It's just a matter of consistency on the owner's part.
Step #1: Give the command "go to your bed."
If your dog actually does go to her bed, make sure to give her treats and praise. Most likely, she'll need you to guide her. Once there, place her in a down position and give her treats and praise. Then release her. "Good girl!" Repeat that five times or so and quit.
At this point, the dog doesn't stay on the bed for more than a second or so. You're just encouraging her to go to her bed and giving her rewards for it. Remember to have some sort of word to release your dog from her bed such as "free!" or "ok!
Step #2: Begin increasing the distance.
Increase the difficulty very gradually so the dog is successful. At this point, you could tell your dog to go to her bed when she is about six feet away from it. Once she is successful from that distance, work from 10 feet away and then across the room.
Step #3: Increase the time.
You'll most likely be practicing this step along with step two. Basically, you want to teach your dog that "go to your bed" means "go to your bed and stay there until I release you."
Ideally your dog would then stay on her bed for up to a half-hour or more while you're doing something else but you need to gradually work up to that point.
If your dog knows the command "stay" then it's OK to use that to encourage your dog to stay. Personally, I don't like to use "go to your bed" and "stay" because "go to your bed" implies the dog should stay.
At first, you'll only expect your dog to remain on her bed for five seconds while you're sitting right next to her. Give her treats and praise. Then release her with -- "free!"