I suggest holding a treat above her head, give the sit command and wait until she sits and then give her the treat. After a while she will sit with the voice command alone. That's what I did with Sheeba anyway. And now when I say sit that bum hits the floor pretty darn quick because she knows she won't get what she wants until she "earns" it. But I have to admit she doesn't always sit other times when she gets excited about walks and play ball time and I have to use a more firm voice and add the word "now," which works for us. She will be two years old next month and still acts like a puppy.
I also like hand signals. I just point down to her bum over her head. That's hard to describe.
For lay down, the hand goes in front of the head pointing to the floor.
Some days I will give commands through voice and other times with hand signals. In my opinion it gives more exercise for the brain to mix it up once in a while.
After a while they get smart and will just do stuff automatically without being asked, like giving paw, etc.
Take your time, and train a few minutes every day in short sessions. And make it fun!