I've taught many a workshop on designing a garden, and I know from experience that it's hard to know where to start. That said, I have stopped dreaming about perfect gardens. It is enough for me, at this point in my life, simply to garden. A walk through Eden takes much more background work than I'm willing to put in. So, very realistically, here's what I do.
First, I take into account my goals. This includes how many veggies I'd like to grow, ideal harvest (data and tables can be found in this quintessential book ), and anything else I want. This includes dreaming big (if I had it in me)
Then, I observe my landscape. This is the part where I figure out what kind of space I actually have, how much sunlight I get, what my soil is like, where flooding or extra frost or wind damage happens, where roadway runoff may have oversalted my soil, where compacted areas might make it hard to till, where in the yard I never go and probably will never go to tend to a garden either, etc. etc. In reality, I do this at the same time as I figure out my goals. This is the "reality check" part of the project.
I combine my goals and my observations in site assessment, which is where I figure out how to meld my goals and my observations. Thus I pick the perfect general site and size for my garden. This includes basic things like "well...I guess I don't be growing good carrots because of how rocky this soil is" or "I can basically do whatever I want. Sweet."
Only after this point do I design my garden. I lay out beds and rows, I make measurements, I draw diagrams and maps. First I plan out the general size, shape and design of the garden at large, and once I have an idea of the general plan, I figure out what goes where (including an easy rotation pattern for future years (rotation refers to not planting the same veggies in the same place more than once every 3-4 years to ensure that pests and disease don't stay in the soil and attack the plants again and also for nutrient replenishment)), how much of each veggie to plant, and other charming features.
Once I plant my garden, I always make sure to have a detailed map of what I planted where, because next winter it'll be hard to remember what your rotation plan was, and you risk planting carrots in the same spot two years in a row out of sheer carelessness. I have a devoted Gardener's notebook so everything (including seed buying receipts and daydreams) goes into one place where it'll be easy to find. A file folder would serve the same purpose.
Always design from broad concepts to details. Knowing where the carrots go before knowing how big the garden will be doesn't make sense no matter how you swing it. Also - this will guarantee that your details will work towards an effective and comprehensive broader concept and actually meet your goals. Sewing together scattered details to form a well-design whole rarely works.
If you want the best design possible have fun and play around with it. sketch out several different designs with thick markers first, and then do several renditions with pencils for details. Move things around, try different shapes and patterns, even if you are absolutely sure where the garden will go, try moving it to learn something new about other potential landscape factors or how you might go about designing the layout of the garden itself. You'll be surprised how much fun it is and how much more effective it is to mix and match ideas from different designs than to just plough ahead with the first great thing you thought of.