This is what I do. I'm sure everyone else will have different things or timescales.
Remember that, worst case scenario, the US will have ALMOST EVERYTHING that you could possibly want, if you leave it behind. What it won't have is your documents, paperwork etc. So about a month before we go I put out ESTAs, passports, insurance documents etc on a little used table. Also money related things, sim cards if needed, and a note to include any prescription medicines we might need. Check it's all OK, in date, valid etc. You can then add to it as and when you might think of things. Don't forget a driving licence with a credit card in the name of the driver.
In the couple of weeks before we go I start to put out things, toiletries, games or puzzles for the plane, teabags (!), sunscreen, plastic bags (always useful), cooling towels, backpacks, ponchos etc. I might not take them all, but have them to hand when I'm packing and can decide then.
In the week before we go it's clothes. Undies (don't forget socks), tops, bottoms, a jumper or two. If you stay somewhere with a washing machine you won't need much. You can buy washing powder in Walmart. Make sure things can be safely tumble dried or you'll have to hang wet things on hangers. Do you need posh clothes?
We've travelled to WDW a lot and I've sort of got it down to a fine art now. It works for us. But Walmart and Publix are amazing supermarkets and will have most things that you need.