Hi dobby, are you planning to plant into a grassed area or start from scratch on a patch of bare earth? If you're planting in grass, cut the grass as low as you can then rake it to create bare patches. It's best to sow yellow rattle seed onto the prepared area in autumn but you can buy yellow rattle as plug plants in april. Yellow rattle is a parasitic plant on grass and will help to create areas for other plants to grow. Find a supplier of wildflower plants and seeds on line; the list of native wildflowers is vast and your choice will depend on the situation of your meadow area. Also try and find out which plants are native to your area as these will probably do better.
Another easier alternative is to clear the proposed area of grass, as if you were digging a seed bed, and sow a wildflower mix but I would still plant the yellow rattle plugs. Hope this helps.