Stuff is first put into plastic bags according to type. I use different coloured carrier bags so I can tell what is in each bag easily. There may be bags for small things within a larger bag.
The pack is fully waterproof and only has an opening at the top.
First goes in clothes. Underwear and socks. (For a soft landing when I drop the bag on the ground.) A different bag for dirty clothes. If I am walking in the sun two wet socks will often be attached to the outside of the pack to dry. If I am walking where there aren't many people I will also attach wet underwear to the pack.
Next goes in hard food such as pasta (vermicelli/cheveux d'ange cooks fastest), couscous, porridge etc. Emergency soya etc. Then a cooking stove (MSR Whisperlight), which goes inside a pan. The pan will be in its lid wrapped in their own plastic bag. Inside the MSR Whisperlite's bag will be a lighter and the pan handle.
Next there are big things, the tent and sleeping bag. Between these, near my back will be the bottle of petrol.
Heavy maps and maybe a guide book go in a pocket inside the backpack which puts them near my back. They will stay in there all the time.
I then push in a silk sleeping bag liner and a NeoAir mattress.
Near the top will go fresh and soft food. Like bread and bananas/apples etc. This is for easy access for a snack. Heavy stuff at the top and near the back. Softer food the highest.
On top will be a waterproof coat.
The lid of my backpack has two pockets, one inside, a larger one outside. In the inside pocket goes my passport, medical kit, toilet paper, toiletries and other small things. In the outside pocket goes my head-torch, GPS (only used in emergencies), camera accessories and waterproof trousers.
On the outside of the backpack will be the sponge mat (=earthmat), and one or two water bottles. These are 2pint plastic milk bottles with hoops that go through the straps. One at either side, then the mat horizontally behind the backpack, or one on one side and the mat vertically on the other side. (Sometimes one bottle may be used in an emergency inside the tent. When I get to a shop I will throw it away and buy another bottle of milk.) If in Europe I may also have a baguette on the outside of the pack, maybe in the mat.
(I have a NeoAir for inside the tent, and a sponge mat (=earthmat) for sitting on while resting and then putting underneath the tent as a protector.)
If it's raining and I put on my waterproofs this makes space for the camera if necessary.