there's two ways to do it, and one of them should never ever be used
the bad way is using the carve option, where you select a brush, cram it through another brush (or set of brushes), tools > carve, and it'll carve out whatever brushes it intersects
it fucks up the brushwork and makes it look really awful and messy
you can also use a combination of the clipping tool
and the vertex edit tool
, to divide the brush into smaller chunks and dragging the corners
that keeps everything easy to work with, and is also less resource intensive than the alternative