Assuming you have all needed materials in your storage system, first you need to explore. Leave a trail to get back home. The landscape needed is normally a plains biome, while it may not be for your project. Once you find a good area, set a bed and a few chest. Put all your materials in that chest. Sleep in the bed so you will spawn there. Prepare the land by removing trees, destroying hills or anything that's needed. Once you have all of this done, Your ready to start building.
Having your build built in creative is the easiest way to know how to build it. When towering up, use blocks taht look way different from your build. Trust me, it'll help. Keep the area light and creeper proof. The safer,the better.
From my experience, working on the same thing for days straight makes you hate it and usually stop it. Listen to music while building or watch YouTube. It also helps to have multiple projects running at the same time. If their's a day where you don't feel like doing the project, don't do it. It will only make you lose intrest.

That's my guide for doing large projects, thanks for reading.