The garden illustrated above is created by chance, it is what nature provides. If you want to create a more specific look, you can add to the backdrop of wilding by planting some wildflowers of your choice. This also speeds up the process as it can take quite some time, (as in a number of years,) for wildflowers to establish themselves naturally.
You can do this several ways.
The first is to introduce established plants, which I have done in a small way: planting Cirsium rivulare 'Atropurpureum' . Although not a native wildflower plant, it looks good in a wild setting and the bees go mad for it, so I added it in alongside the steam. This is also a perennial, and so graces the garden each year.
Alternatively, you can buy small plug plants or seeds and introduce these into the wild area. If you are doing this for the first year or so you will need to clear grasses and "weeds" from the immediate planting area to give the new plants chance to get established. Some plants, such as Dipsacus fullonum the common teasel illustrated centre, are easier to establish because it is robust, some would say a pest, but it's a pest the pollinating insects and goldfinches love.
Or you can just leave the area and keep an eye on what grows. Thin out unwanted visitors and keep the balance of weeds and flowers. If you feel that not enough of your wildflower favourites are seeding themselves naturally, give nature a hand and sow some perennial wildflowers. It is important to distinguish between annual and perennial wildflowers. The planting scheme, wilding, is based on mainly perennial flowers so that it is easily sustainable year after year.
Wilding is different to creating an annual wildflower patch or meadow. If this is your preferred route, you will need to clear the area and keep it weed free, which is very difficult in a wild garden area. Many of the wildflower meadows in the large show gardens we visit are created to be just that, an annual display. They are made from annual seeds which will die back each year. This method of creating a wildflower garden is the most time-consuming. It looks casual but is high maintenance. Tips and growing instructions for creating a wild flower patch.