The problem with planting in grass is that at first the grass can easily out compete the flowers, then you get "rank" weeds muscling their way in. Also a lot of garden soil is just to rich for wildflowers.

The best result is obtained from either stripping the turf, or you could cover it up until the grass dies, or use chemical's.
You should use a combination of annual and perennial flowers, and don't expect too much in the first year.

I've just recently made a new bed in the garden and have used bee and butterfly friendly plants and am about to sow wildflowers in between until the other plants get going.
I'll have to do it at night though, so the hen's don't see me and scratch them all up again

My problem with planting anything that grows is rabbits and had a deer come right up to the house to eat the red tulips, left the pink ones in the same tub for some reason. I don't think a raised wildflower bed would have the same effect.