Are you planning an annual meadow or a perennial one as there needs are very different. I have both but the annual one is better on my heavy clay soil.
If your lawn has been cared for then leaving it to it's own devices probably won't give you the wildflowers that you desire as rye and similar grasses used in lawns are too 'thuggish' for wildflowers. Adding seed will help if you rake the lawn aggressively and create some 'bold' patches to sow onto however you may not get results as quick as you had hoped from my experience.
I have tried it several ways but found the best was to strip the turf and sow fresh, this has certainly proved the most reliable as far as perennial wild flowers go for me, however I did sow some annuals the first year whilst the perennials grew as they will generally flower best from their second season on.
The other thing I did was researched what type of wildflowers were native to my part of North Somerset and where possible sowed these species.
Another tip from me is to also grow some perennial wildflower plugs from seed and plant these in any gaps for the first year or so and they will soon mix in! If you end up not needing them all plant them in large pots for a mini meadow or what I do is plant them in between herbaceous perennials in my pollinating border and they add a nice twist.
Plant annuals in the first year so Perennials have a season to get going...
Hope that helps