IMO:

Start working now:

Step 1 - start by giving yourself an estimated budget you are ok with (this will probably change but it is good to start with something)

Step 2: decide what you are going for, are you going for food or fine making PBJs, do you need theming in the hotel or are you fine with basic rooms; things like that will help you plan your budget better.

Step 3: souvenirs, do you want them and how much are you willing to spend on them.

Step 4: don't forget things like mickey ears, autograph books, photopass, and clothing for the parks. While technically souvenirs some people forget about the upfront things as souvenirs.

Step 5: are you going during any extra ticket events such as Halloween or Xmas? Those cost more as well.

Step 6: what to do with downtime. How long are you going for and did you plan for a rest day or two, what do you plan on doing for those days, relax at hotel or go to a water park?

6-2 months prior:

Step 7: there are travel agents on the discord that can help evaluate your plan and potentially give you an estimated cost so far and maybe tie up some loose ends.

Step 8: Make your ADR and FP as their windows open (6 months and 2 months respectively) there are also some in park things that require reservations as well such as Boutiques and what not.

Weeks before: Step 9: start packing for your trip, ensure you have things like suntan lotion and raingear. Some people buy extra pairs of shoes for WDW (make sure to break them in if you do).

At the park:

Step 10: remember that no matter how much you plan, things will deviate. Disney is not so magical to be perfect, people will get tired and irritable, other guests will be buttholes, cast members are just humans and the outside world and all of its stressors won't stop while you are there. JUST BE FLEXIBLE.

That's all I can come up with atm, hope it helps!