Outline the digging pit with 2x4's so it is clear where the boundaries are. Be sure it is in a spot where the dog will want to be (i.e., it will be shady in the summer). An area of about 3 x 6 is sufficient. Dig down to a depth of 2 feet and mix in some sand.
"Seed" the legal area with dog cookies, smelly treats, etc., while she watches ... act excited about it. Don't plant them too deep to begin with. While one person plants the treats, the other should hold the dog back so she can't get to them. Then, tell her "dig in your pit!" and praise her enthusiastically when she starts scratching. You can bury the items deeper after she gets the idea. If she digs up a treat, she gets rewarded by getting to eat it. If she digs up a toy, play with her ... chase, fetch, whatever she likes to do, and then bury the toy again. Last thing at night, bury a couple of treats and first thing in the morning, go out and tell her to "dig in your pit." If you give her a large bone, tell her to "dig in your pit" and hopefully she will go bury it there.
Keep the legal area damp to encourage easy digging and keep hiding things in the pit.
Until she has learned that this is the only area to dig ... never leave her out unsupervised.
Outline all of the places she isn't supposed to dig in with short fencing, or even string held up with sticks ... it is just something to make a clear boundary. I'm assuming you have already taught her what "leave it" means since that is part of what I send to every adopter (if not, see: "Leave It"
on this website). Spend Saturday out in the yard with a good book and every time she approaches the boundary, say "leave it!" If a single paw goes over the boundary, say "oops" and put her inside. After five minutes, try again. If you say "leave it" and she pulls back her paw, say "good!" and give her a treat.
After she is reliable with you in the yard, you can set her up to really reinforce it. Drip something tasty in the forbidden areas (bacon drippings; chicken broth). When she starts to investigate, say "dig in your pit" and run with her to the pit (have it seeded with something good) and really praise her when she starts digging.
Then, you can start leaving her in the yard on her own, but keep watch out the window. If she starts to cross the boundary, rap on the window and say "dig in your pit" and run out to direct her to the pit.
If all else fails, you can circle the forbidden areas with an invisible