The room situation is that two rooms are really quite bad - bags of random clothes on the floor, piles of books, piles of correspondence etc.
I'd start with the worst rooms and just focus on categorising and containing everything unless you can clear significant space in one of of "better" rooms in a short time (ie an hour or two). If you can, then I would clear that room so that you have space to move things out of the worse rooms as you work through them to give you the space to work. If not then I'd just go straight to the worst rooms and get stuck in.
Before you start make sure you have plenty of bin bags and some boxes/cases to put stuff in. If there are any big items that the homeowner 100% does not want to keep then just get them out of the way ASAP, even if you just throw them into the garden/back yard while you work.
First take a short time - I generally aim for no longer than about 20/30 mins but it will depend on the space/size - where all you do is find and throw out any obvious rubbish - old envelopes, food wrappers, receipts, empty bottles etc. Don't spend too long on this, it's literally just to gain you a little bit of space, get rid of rubbish and get the process started.
Once you've done the rubbish sweep you will have generally at least looked over everything so you should now be able to start compiling some general categories for the stuff - clothes, paperwork, toys, books, miscellaneous etc. I then generally go through the room with a focus on one particular category starting with the biggest - so if there's lots of papers then I'd go through the room focusing on pulling out all of the papers and putting them in one big box first; then once I had done that I'd move on to the clothes and pile all of them into a box/bag etc, if I found more papers in a clothing pile then I'd take a second to put them in the box but I'd ignore any books/toys for the moment.
Once you have worked through the big categories then you should, in theory have a few large piles of like items ... Once you have them categorised you can decide what is to happen to that category and then deal with it (even if it is "books are to go on the bookshelf but first all the crap on the bookshelf needs to just go in a box so the books can be put away and the crap dealt with later if there is time, but if not then they can stay in the box marked "bookcase crap")
If you work with this general process - rubbish, categorise, contain - through each area then you should be able to get some sort of system in place. You might not get everything done but you will leave the homeowner in a better place to continue (they may not be able to sort the whole room but if you get the roomful of clothes all condensed into 10 binbags then they can sort through one bag at a time to decide which items to keep/toss/donate from that bag and then deal with it in smaller, more manageable, chunks)