The original poster is looking for advice on PoA, in plain English.
It has to be done officially, with a legal document by your lawyer. Without that, whoever you choose to have power of attorney might not be accepted if she made a decision on your behalf when you were not able to decide for yourself.
After all, they could be just anybody saying "But she told me she wanted me to do it this way. Give me all her money and don't bring her round her if she is very ill." when you what you would have wanted them to do is to use your savings to pay for a nice care home for your last years.
If you don't have a proper Power of Attorney certificate, get that now. A lawyer will guide you through it. The certificate will be stored until it is needed and then be activated. There is no need to worry that the person named can immediately withdraw everything from the bank and vanish - it won't be activated while you are still running your own life, it has to be activated first.
The important thing to remember is that if you decide to get the Power of Attorney too late, when the lawyer thinks that you have already become too confused to know what you are signing, he or she will refuse to draw it up for you.
So the time to do it is when you are fully aware of what is what and you are able to say clearly who you want to act for you and what kind of thing they are to have control of, health decisions, money ones, or both.