A meadow is a field of grass allowed to grow each summer for hay. Since it is permanent grass, seedling find it very hard to compete with the much taller grass, so most of the plants are perennials, lasting from year to year
A “wild flower meadow” in the seed-purveyor’s sense is a collection of annual (dying off each winter, growing anew from seed each year) flowers which in the wild are weeds of corn fields, ploughed each year, giving seedlings the clear ground they need to germinate. There are clues in the names - corn marigold, corncockle, cornflower etc. This is what @Skyellaskerry is mimicking.
Assuming you are after a meadow, ie with grass, the point you need to be aware of is that grasses are really well adapted to take advantage of high nutrient levels. So the poorer your soil, the better chance the other plants have of competing. So if you have a bit of lawn that’s been well cared for, the best way is to strip off all the existing turf (much easier than it sounds) and either sow a meadow mix (which is made up of grass and perennial wild flowers, with usually a few annuals to keep your hopes up during the first year) or buy some wild-flower rich turf.
If your lawn has been less well looked after and already has daisies, clover etc, you can plant perennial wild flowers into it, and hopefully they will slowly increase.
People will tell you about yellow rattle, which is parasitic on grass and reduces its vigour. In my experience, it’s hit and miss in the lowlands. The grass gets away first in the spring, and may mean the yellow rattle doesn’t get enough light and fails to flourish.
The other thing to be aware of is sunshine. If the grass is shaded, it’s difficult to establish a meadow. My shaded grass has bugle, forget-me-not and germander sppedwell in it but not much else.