I do use diagrams as a part of my process, but it's just a tool - not something I feel like I have to stick to. I usually spend a far amount of time just "listening to what the space has to say to me" as I call it. I do a bit of research on the local climate. After I do my initial assessments I make lists of plants for each microclimate around the space I'm gardening in. Then I make a rough diagram of how I'll arrange my plantings. I usually reserve a lot of space for just broadcasting a bunch of mixed seeds. I've had the best success broadcasting seeds and letting them grow wild. That gives me an opportunity to really see what grows best in which of my areas and which ones don't do well at all. Then the next time around, I don't bother with the ones that don't thrive in my space.
I put more care into where I plant perennials. Since they will stay in place rather than dance around the garden like my annuals will, I want to make sure I get their planting location right. They are the first things that go on my diagrams. That has been even more important for me because I've always had small spaces to work with. The smaller the space the smaller the margin of error you have. Of course, other things might go on a diagram: where rabbit cage, pond, rain barrel,
compost piles, etc.