When she's gotten the idea, you can turn the post right-side up again. Scratch the post with your own fingernails again or use the string to entice your cats to use the post in its upright position. If she stretches up on the post but doesn't scratch, gently stroke her back which will stimulate the scratching reflex. Let her think everything she does is her own idea. If she's a kitten though, don't use catnip because kittens don't react to catnip when they are so young.
Cats generally like to have a good scratch after sleeping or eating.
Make sure the scratching post is convenient for your cat to use. Don't hide it in some corner or in a rarely used room. IF your cat has been scratching on a piece of furniture, like a sofa or chair, put the post near it. Don't put her paws on it and force her to scratch though-just scratch it with your nails. When she uses the post successfully, praise and pet her.