Start eight weeks out and work room by room
Do one room at a time and finish it before starting the next. Half finished rooms create the impression of chaos and people give up. Begin with storage spaces because the loft the garage and the spare room hold the most that nobody wants.
Use four piles not three
Keep sell donate dispose. The fourth pile matters because without it people put broken items in the donate pile and charities end up paying to dispose of them.
The twelve month rule with exceptions
If you have not used it in a year it goes unless it is seasonal genuinely sentimental or expensive to replace. Be strict about the first category and generous about the second.
Sentimental items last
Leave photographs letters and keepsakes until the end when your decision making is warmed up. Photograph bulky sentimental objects and keep the image rather than the object where you can.
Deal with the piles immediately
Book the charity collection list the sale items and take a car load to the tip in the same week. Piles that sit in a hallway for a month end up back in the house and then on the van.
In short
Eight weeks one room at a time four piles and clear each pile the same week you make it.