Hey Jenny4 and Doodles,
You're on the right track by using a strong firm voice,
Keep it up and always show her the mess but more importantly,
#1) Get her close enough to smell what she has done.
#2) Associating the mistake with a Firm no and smell will register more
quickly with Doodles as opposed to seeing the mess.
As you said, it's quite hard catching them in the act but I'm sure
you're doing just fine.
#3) As soon as you notice the mistake or catch her in the act, take her straight to the litter box so within time she'll learn the correct place to poop.
1 more thing, (the most important), Always treat Doodles to her favourite biscuits and give her loads of cuddles when she does show signs of wanting to poop in the litter box or outside

All the best.
Please stick around and enjoy the forum my friend.
Sarah