Is it to do a poop or a pee?
If she's pooping on the ground and her litter box is clean, she might be doing it for attention, or to annoy you. I had that problem with my kitten for a while. If she was naughty and we sent her into the garage for time out, we'd come in to see her hiding behind the chairs, watching me and waiting for me to find her nice little poop. Also, if she has pooped on the floor before, she will be able to smell it and will think it means she's allowed to poop there. Make sure you cover all smells. Vinegar is good.
Put down cardboard around her litter box so she knows it's just in the litter box that she is allowed to go to the toilet. If she can't scratch over the poop she will get annoyed, so I've found my cat doesn't poop on cardboard. I did have newspaper down, but she's just pull it up, poop, then put the newspaper back down, so that wasn't teaching her anything.
When you find the poop, show your cat it and hit her (gently) on the butt and say "NO!" Then pick it up and put it in the litter box. Encourage her to scratch it over. When she starts covering it over, pat her and say "Good girl", things like that. I also like to give her a little treat when she does that. I also give her a little treat if she does a poop in her litter box without me having to watch her. She'll associate treats and cuddles with pooping in the box.
This is only if it's poop I guess. Sorry I can't help more. It's just that this is something I've had a problem with, so it's all very fresh in my mind.