I have never tried this before, so I decided to learn how to do it. Below is an animated gif of the situation at the start of the first five turns of the game. Unfortunately, I didn't have the screenshot option selected in the game that automatically takes screenshots of the entire map at the end of the turn so all I can make are screenshots from the start of the turns.
If you watch closely, you can see my strategy playing out in the animation. In the northern areas, I am doing a slow withdraw as the situation warrants. In the west and northwest, my units are fleeing to Warsaw. In the southwest around Lodz, I am trying to slow down the Germans by throwing as many units as possible in front of them. And in the south and souteast, I am trading ground for time.

Note: if you're interested, the previous test post was so I could test out how to post an animated gif.