I'm a bit out of my depth here but I'll try to point you in the right direction anyway.
First decide whether it's an Ordinary Power of Attorney or a Lasting Power of Attorney that's required. (See my first link below)
The advantage of OPAs is that there's no requirement to register them with a central authority (saving on some fees). The disadvantage is that they need to be created
individually, which means that you'll need a competent adviser. (That would usually be an expensive solicitor but your local CAB might have access to the services of someone who could do the job at a reduced rate or even without charge).
LPAs have to be registered (incurring fees) but, because they use standard forms, there is no need to use a solicitor (thus saving on even bigger fees).
See here
http://www.adviceguide.org.uk/england/relationships_e/relationships_looking_after_people_e/managing_affairs_for_someone_else.htm
and here
https://www.gov.uk/government/collections/lasting-power-of-attorney-forms