From what you've said, I think the first step is to manage your own expectations. Even though she has asked you to help, she may not want you to throw away everything or maintain the cleared rooms to a standard that you would like
This with bells on !
My DParents are 82 this year. My DMum has always been untidy and looking back , always been a hoarder. I blame her childhood , she was a child of rationing times which I know doesn't automatically mean : Child Post War= Adult That Hoards but it goes a way to explaining.
Some of her happiest memories are things like her Mum making a coat for her with scrap material and meals from nought (and they were nice too)
She never let us throw things out and always wanted "A tidy house" for birthday or Mothers Day but no matter what we did , it never lasted and it became our 'job'
So, now when I go to help, I know I'm fighting to make her let go of things.
She also keeps any receptable (boxes , baskets etc )just to fill with things .
Will your Mum let you have free rein or will she do what mine would do ( say "Right" and insist the bag is open so she can ransack through it and claw everything back) It is tempting to say "Stuff It" and just tip everything on the floor.
I am going up to DParents in a few weeks. My DDad room is very tidy , he can tell you exactly where everything is. Though he does pick something up that i have put n the 'bin' pile and say "Oh look she's throwing this out" . I give him a 'look' , tell him to make me coffee and that he's being the Opposite of Help.
Three Bags:
Bin (rubbish)
Recycle(paper/plastic)
Charity Shop (but only sellable things and fabric rags that they take for scrap)
I go through each thing , sometimes she will grab it back and say "No" . If its something dear to her , I pick my battles.
Something like bedding, I go through into sets and get her to chuck out the tatty ones. She had loads of unused and unusable .
If she has 2-3 of something , I get her to pick one *things like a peeler or measuring jug , she doesn't need 3)
Get the bags out of the house Straight Away . Lock them in your car or a shed . It gives you room and gets them out of her mind.
I destroy all the boxes etc as soon as they're empty . DDad is an enabler "Your Mum might want to keep that" .. No ,
They do try to give away rather than put in the bin, but some things are not recyclable and it isn't fair to give the Charity Shop the job .
If you need to go to the dump, will you need her details ? Most will only let you if you are a house holder . Best to contact them to find out.
My Dmum has 2 knitting machines and 3 sewing machines plus endless bits of material that she is adamant she will sew (she won't). The knitting machines are under the bed .
My task is to get the majority of the material out (I left her some things she said she was going to sew . She won't have done . She can't even hold a fork let along a needle
)
Good Luck !