use treats. use a clicker or a marker word. i use the word "yes". say yes then immediately give the treat. give her a treat when she sits correctly. next training session ask her to sit and wait a few seconds before giving treats. rinse and repeat and add more time.
after your dog can sit to your desired duration then you can now ask for a sit and take one step back. if you dog stays still then say yes and immediately let her come to you for a treat. if she gets up to follow you then just grab her leash and bring her back to the starting point and ask for the sit again. you can then take more steps away from her. enough sessions and she'll learn that her release word is yes or a click of the clicker and will continue doing that command until released
p.s. i personally dont teach the word stay. if i ask for a sit the dog needs to sit until released. if i ask for a down then the dog lays down until i either give him another command or release him. "stay" should be understood.