Using Power of Attorney to manage someones elses tax

I have successfully registered my mothers LPA with HMRC. When I asked how I see her self assessment, the response was she needs an account. Currently she gets an accountant to submit her tax form (expensive and unnecessary). The process for me to get access is unclear, the indication is that my mother needs to set up an account, which defeats the purpose of an LPA. When I registered the LPA by phone, they said all the LPA does is redirect HMRC mail to me. Can someone let me know what steps I need to take to get access to my mothers self assessment information for this year (so I can pay the tax she owes), and to be able to make self assessment returns for her in future years.

Hi David,
If you have LPA on your mother's record, you can help her set up her personal tax account, which can be used to file her Self Assessment return here:
Personal tax account: sign in or set up 
For more advice, please contact us by webchat or phone here:
Self Assessment: general enquiries
Thank you.