Using green manure?

I've grown mustard greens and they're just about looking like they're ready to flower, so I've pulled half of them up. I grew them mainly because I read they can help keep powdery mildew under control.. I had loads of this last year on my courgettes and pumkins

Do I chop them up in to tiny bits or clump?

How deep in the soil should I put them? Is it ok to plant seeds in them or should they be higher or lower than the seeds?