Trial and error is great, and pretty much what all gardening is, you just get more informed about the likelihood of what you are trialling working.
Imo, you really need to cut in a bed if you want to make it look intentional. Just get a spade and cut out the turf in a shape your like. You could go long and rectangular along the edge of your garden (like a border) or a wide circle around the rosemary. Or even a wiggly border.
Chuck the turf in your green bin (I'm assuming you don't compost), and dig into the soil you've uncovered to break it up a bit. Remove any big stones etc. You can look at doing some soil improvements here if needs be, but I'd be tempted to just work with what you have for now unless it's awful.
Then, plan your space a bit. Big stuff at the back, small at the front. When you buy plants check how big they are going to grow and give them space to mature into. Your bed will look a bit gappy for a couple of years, that's ok.
Alternatively, have a look at something like garden on a roll or border in a box (I've never used them, I think b&q also have similar), that will give you a plan and the plants in one go. A bit pricey, but trial and error can also get very pricey too!