Hello
I have a wildflower patch. Honestly? You need to get rid of the grass - it gets everywhere and completely overtakes the wildflowers. I have sown seeds and planted a few wildflowers myself. I've also grown foxgloves from seed and planted those - so not completely a wildflower garden.
Do you want a perennial wildflower garden or an annual one? If the first then you need to make sure that the soil quality is poor, to stop grass from getting in. This means digging up the grass and the top layer of soil. I know it's a total nuisance.
It's worth bearing in mind that wildflowers are weeds. Stuff like teasel is what I pull out everywhere else. But here I love it and keep it.
Realistically, you are rather late for it this year. I think that if you are sowing seeds, you can do so in the autumn for a spring germination - not if you are on heavy clay though as the seeds will rot.
At the end of August, I get my sickle out and cut everything down to 30cm and let it lie for a few days. Then I transfer it to the waste compost bin. This allows the seeds to fall but taking the stuff away means the ground doesn't get fertilised with the cut flowers.
I buy seeds from these people. They have different mixes, depending on where you are / what sort of soil you have.
Having said all that, I only know what works for my wildflower patch. Other people may do it differently / have different conditions etc.