Start At The Beginning
Not always. Most well-defined creative projects (making a picture, an art work, writing a story or novel) usually, but not always, benefit from starting at the end and working backwards. Working with other people's image files all day, I am constantly amazed at how many people don't actually have any idea what they want to end up with. I'll be handed a square format image, and be instructed to: "make it a 16x20, but don't crop anything out." With the advent of the "four-thirds" format in digital cameras, none of the traditional print sizes fit the new cameras. Which is OK, IF you specify a crop, or figure out a size that does fit (multiply the short dimension by 1.33 or the long dimension by .75 to find a size that matches your format.) You were sick that day during math class in grade school? For three years? Try a calculator!
Of course I could cheat and stretch the image digitally (not recommended), or even "clone in" some extra sky or background to fill in a blank space (done far more often than is admitted to.) In the end, my job is to make people happy. So I start there on my tasks, and work backward.
Of course, you can just start some projects to see what will happen. But that would be play, not work, sort of like blogging?