There’s a couple approaches to this, and I’ll outline them here briefly.
The first approach is to take a tractor of some sort with a mower and rake attachment. Using the rake and front bucket, clear out big stuff as best you can, getting rid of rocks, logs, etc. Then take the mower and cut it all down to six inches or so. Much shorter than that and you can damage root systems and risk erosion.
Once it’s short, then you can take a discing attachment and till your field. Once it’s tilled and all the current vegetative growth is disc’d under the dirt, you can spread new seed and let it grow up.
Seems like a painful and tedious approach to me, and definitely not my preferred one.
Another approach is somewhat similar to the first. Rake and clear the pasture, then mow it. But once you mow it, just leave it be. The grasses grow faster and better than the brushy stuff does, and so by mowing it all down a few times you’ll pretty much knock out the bad stuff and be left with grasses. You can still seed with a good pasture mix if you like which will just help things along.
This is definitely better in my mind. The big issue I have here is that if you miss rocks or logs and run over them with your brush hog or mowing attachment, you’re in for a bad day. And, well, it’s pretty much guaranteed to happen if you’re prepping a bigger pasture.
My preferred approach is pretty lazy. Don’t do anything major at all. Fence off your pasture if it isn’t already fenced, then use temporary fencing to create smaller paddocks.
Then put your cows in the paddock and let em sit for a while. They’ll eat the good stuff, then eat some of the junky stuff. And they’ll poop … err … fertilize all over the place too. Sheep would work too, but don’t use horses. If you don’t have cows or sheep, borrow some!
Once the cows have sufficiently knocked stuff down, move em to the next paddock. Somewhere in there you’ll want to seed too, giving the whole system a helping hand. Repeat this a few times, and you’ll have some extremely pretty pasture. Just let nature’s designs work the way God intended, and you’ll be fine.
If you REALLY want to, go ahead and clean up the rocks and logs. I probably won’t bother…
I hope this helps … holler if you have any questions!