Leaves alone may not kill the weeds and grass, may well just provide fertilizer for them.

You need to deprive the growing weeds and grass of light to kill them and starve their roots. This is easiest done with 6-8 layers of newspaper, so start saving newspaper now to lay in the spring.

As soon as the area greens up in the spring, gather some friends and have wheelbarrows, water, a truckload of fine compost and your shredded leaves ready. Have one or two people lay the newspaper over the area, followed by a someone else adding a light sprinkling of water, then shovel on 2-3 inches of compost and finally cover with shredded leaves. In six weeks you can plant by digging through all the layers. I would recommend plants or plugs not seeds as it is easier to recognize and pull weed seedlings compared to wildflower plants. Seeding is more difficult as you will need to scatter the seeds on the compost and not cover with shredded leaves which will leave the area open to weed seeds which can blow in. As seedlings, it is very difficult to tell weeds from desired plants.

I would avoid the commercial variety seed packs of annuals and perennials unless you get it from a native plant source like Prairie Moon nursery. You will need a nurse crop for the first year and to follow the instructions provided. The annual/perennial non native mixes usually only look good the first year.

Given that most meadow plants take up 3-4 sq feet, you will need about 100 plants to fill the area. You could plant 30 to 50 plants early next summer and then divide them in two to three years time to fill in. At cost of $3 to $4 per plant, it is affordable and much faster and better controlled than seed.

You can kill plants with layers of leaves, but it will take a lot more than three inches of shredded leaves. First I would mow the plants, then I would add at least two feet of unshredded leaves immediately, which will be only about 6 or 8 inches deep by spring. On top you can add some shredded leaves, which look a little nicer, but that is optional. This should kill most of the plants that are currently there. The ones that poke through can be cut off at ground level with a shovel, the leaves replaced over them, and they will die after a couple of such treatments. I think if you can put enough leaves on the site you can get pretty complete kill of the existing plants by next summer.

You have a pretty small area, so I would strongly recommend you consider using plants, not seed directly in the ground. First, you can't seed over mulch, so you'd have to wait for the plants beneath the mulch to completely die, then remove the mulch to seed. This wouldn't happen until late next summer, I suspect. Also, you have to deal with weeds sprouting from seed. With plants you could start next spring, placing each plant into the soil beneath the mulch. You could start your own plants from seed and then put them in the ground, which I think will work better than sowing the seeds directly in the soil.

I would not add any fertilizer to the soil, unless you plan to weed the area as you would a vegetable garden. Fertilizer will cause a lot more long term disturbance to the native plant community than Roundup would. Fertilizer will encourage weeds, and the effects may last for years. Fertilizer will also encourage the natives to grow uncharacteristically large, and they may tend to flop over as a result. Also, I would be careful about adding concentrated acid to the soil, as this may have long-term impacts. You didn't mention it, but definitely don't try killing the plants with salt - this will sterlize the soil for a long time.

One option you didn't mention is tilling the site. If I was going to start in the spring I'd dig or till or plow to kill most of the plants, then deeply mulch to prevent the plants from resprouting and prevent seeds from sprouting. Then I'd put in plants through the mulch. If you're starting now or are not in a big hurry, it is easier to simply mulch and wait for the grass to die, but tilling will get you to the same place sooner.

You will probably want to keep mulching for the first few years to keep weeds out. As the plants fill in, there will be less and less room for weeds, and after a few years you can stop mulching and expect only a few weeds here and there, which you can simply pull.

First of all, what kind of garden are you planning? Wildflowers? Native plants?

Also...do NOT till up the ground. You will expose tons more weed seed and it will be too happy to germinate.

I agree with previous comment--Prairie Moon is excellent. If you go with seed, they have mixes for various situations. Either mulching or tilling will get rid of the weeds unless you have something miserable such as Canadian Thistle. You will have to till regularly over several months. Covering the area with black plastic works well.

I seeded a 400 sq. ft. area that was lawn some years ago after tilling for 3 months. I seeded in November. The next year I mowed the site 3 or 4 times with a hand mower set on its highest setting. Annual agricultural weeds will actually give your seedlings some protection if they aren't allowed to totally shade out the tiny plants. By the second year you may not have to mow or you may mow once, but no lower than 12 inches. A weed whacker works well for this.

The downside to seeding, as noted, is you have little control where a given species shows up, but Mother Nature will craft a plan if you do it this way. A third option is to seed shorter native grasses such as Little Bluestem, Dropseed, Sideoats Grama, and then to live plant wildflower species later wherever desired. If you grow native prairie flowers without grasses, you will likely be spending more time battling weeds. The grasses are beautiful and they are natural companions for prairie flowers.