Go to my link in post 13 and start filling in the LPA forms. You can easily do them online, save them, go back later and continue. Remember that all a solicitor does is input the info you need to give him and charge you a premium for the privilege. You need to have names and addresses of everyone involved, donor, attorney(s), certificate provider, witness, persons to be told. Figure out how many attorneys (just one is fine) and if there is going to be a replacement attorney in case you fall under a bus. Choose the certificate provider with care. Best is a friend or neighbour who's known her for at least 2 years and will certify she still understands what she is signing in the moment.
Fees are low - £110 per each LPA which halfs if the donor has less than £12,000 a year coming in, or is entirely free if she is on qualifying benefits.
Do it as soon as you can as capacity can diminish quickly. There are lots of guidance notes but if you are feeling unsure, ask your local CAB, Carers Centre, Age UK or Alzheimer's Society to help you fill it in for free. Plenty of TPers, myself included, have done this ourselves, so you can always ask us questions too.
Obviously, it needs to be said that if she has lost capacity to agree to this, your only other route is obtaining deputyship. This is more difficult, more expensive, requires you to provide yearly accounts and is usually only granted for finances and not for health. So please try to obtain LPA before it's too late.