It's pretty similar to how I approach web design.
How will it be used? Where will it sit? What conditions surround it? What is the style of the rest of the house? What function does it serve? How does it improve the lifestyle of the people who use it? Build the right thing for the right reasons. Anything I build should last at least 100 years, so physical and aesthetic durability are crucial.
I try to get real world measurements if possible, but if all I have is a photo I'll do dimensional analysis in Photoshop. Next I create a 3D model of every component part. This helps me understand how everything is going to fit together.
This is where I push the design, try different variations, and question everything.
For larger projects I break every operation down into a gantt chart to organize my limited time in the shop – separating power tool and hand tool operations. Then begins a slow and steady march as I get things into their rough dimensions first, gradually working my way to the finest details. I'll test each joint as I go and do several dry-fits before adding glue.
Slowly, steadily, and safely.
The finishing phase of every project can take almost as long as the initial construction, but it's worth the patience and care. A bad finishing job can spoil the best joinery. The finish is the first thing people see and touch. It's their first interaction with your work.
Let it be a good first impression.
"Have nothing in your house that you do not know to be useful or believe to be beautiful."
2 years in planning and design, and 10 months in construction — this dresser represents the zenith (so far) of my woodworking abilities. I chose the most challenging and time-tested joinery, used only the best materials, settled for nothing short of perfection, and gave it everything I had. My wife deserves no less.
From design to installation this took about 12 months. I purposely chose several techniques I had never attempted before simply to gain new skills and raise the difficulty a bit. I learned a ton along the way.