This link will be an excellent resource to help get you started.

Do not amend your soil, native plants don't need it. The best way to get rid of thatch is to do a burn, if possible. I think raking would probably work, but that's a lot of raking! April

Here is a link that might be useful: The Ladybird Johnson Wildflower Center

I wouldn't kill all the existing vegetation if there are desirable species growing there either.

Over an acre is alot of raking..., though it would work. You could also do a burn (carefully) ... or you could mow it short and do a slit seeding and keep it mowed for a couple years to give the seedlings a chance to grow.

If you have a decent cover of mostly native plants, then you can add new flowers by adding plants or seeds of the new things you want. For each new species pick an area and either sneak your new plants in among the existing ones or clear an area (preferably an area that has plants you want to get rid of) and plant a small patch of the new ones.

If the new plants are well suited to your site they will either survive where you placed them or spread around the area. If they aren't well suited they will die out after a year or two.

I agree, I wouldn't kill all the existing plants unless you have mainly undesirable plants. You seem to have mostly desirable ones.