I was quite blunt with my parents about it in some ways but it moved the process on quickly and it was relatively upset free. They both have AD but have no insight into this but I was concerned that they were going to lose capacity so needed to have the POA's in place. I printed off all the forms and filled in as much as I could before involving them then initiated a conversation about 'in the future what would happen if you were incapacitated who would you like to make decisions for you'. They immediately said me, no question about it. I then said that in order for that to happen they needed to sign forms now and explained about POA. I also arranged for the GP to call round to certificate it whilst I was there...and my parents were fine about it. All documents signed and done quite quickly. They didn't relate it to their current medical conditions...but thought about it as a future plan if they were to have a stroke or something of the like.
If I had left it to them or talked about it for too long I was afraid it wouldn't happen or it would be too late. I registered the POA's straight away. A year and a half down the line I have now had to start using the POA's and very glad they were already in place.
It's hard but I would take control of the situation and try to take the stress out of the situation by doing as much as you can yourself and sorting it quickly so she doesn't have to think about it all too much.
Good luck with it