Delurking thanks to summons from Sally..
Once wool is scoured ..which is a whole other question...
Basically.................it depends what you are aiming for.
Wool batting as used by quilters is a semi-felted product, needle felted basically. Large machines for needle felting large bats can actually be found at Winghams. They have a rate for using them on site iirc. The far less expensive and rather more home made approach though can come in one or two forms.
1st style. Old english style of making quilts. These were not the pretty fabrics sewn together style that developed into what we think of as quilting now. the Duvet style is a sewn collection of separate pockets. Literally sewing small pillows of stuffed fabric together. by hand sewing individual pockets each independant pocket can if needs be be replaced or mended or refilled in the even of any nasty clumping of fiber over time..Produces a duvet.
2nd style and probably the way most would approach it, several layers of carded fiber laid at right angles to each other until a large enough and dense enough area is achieved, even hand carders, with enough layers/overlapping will produce a workable batting (batting being the quilting term as opposed to bats, the spinning term) . Gradually producing enough that when layered into a fabric stuffing fabric sandwich produces a quilt. It is important to remember that the stuffing will take up area, so leave plenty of margin for completing your quilt. Similar to making a whole cloth quilt. A good 6 inches all round.
Also, as with any quilting, over time the batting will shift. like modern hollow fiber duvets the stuffing would gravitate into dips and hollows. Quilting..the actual hand stitching of decorative patterns on fabric batting fabric sandwiches aims to minimise that shift in the stuffing fibers. Modern factory produced quilt batting will tell the quilter the maximum distance of any quilting design.
For the non quilter, my suggestion would be a mix of both methods. by making a thick layer of carded fiber, as evenly distributed as possible. Making linear pockets and a batting wide enough to fill each. Your batting should be approximately 1-2 inches in depth for loft, unless you want a particularly heavy quilt. Remember the fiber will settle and compact over time.
IE ..Lay 2 fabric layers each with a 6 inch margin wrong sides together, Machine sew a vertical centre line before adding 8-10 inch width of home made batting, sew a second vertical line that catches all three layers and becomes the edge of the next pocket. Working from the centre to the outside edge to create the desired number of pockets. When vertical pockets are completed over sew through all 3 layers, at right angles to make the square shapes we associate with duvets. For belt and braces, tieing stitches in each pocket. Think a big tacking stitch that is then tied to secure the fibers from shifting too much.
The more you stabilize the fiber the longer the duvet will last. So a little needle felting of each batt would be worth while . The limiting factor in any such venture is going to be the throat of a household sewing machine, so it is worth working from the centre out towards the edge of the duvet. Remember too that wool is both heavy and warm.
When it comes to washing you are going to want to avoid felting the fibers in your duvet. So wash as if your most precious cashmere jumper was in a huge pillowcase. Washing is not something I'd want to be doing to a home made wool duvet very often. You really want to be treating these in much the same way our Edwardian great grandmothers treated duck down comforters. In many respects you are making the same thing.
One final quilting method that might be worth giving a thought to is a form of Trapunto, I originally knew it as Spanish Quilting but have no idea if thats a common name. Again pockets are sewn and then stuffed with the equivalent of the fattest wool roving/lightly spun fat yarn, you can produce. here you would be aiming at a very decorative pattern of pockets, tramlines that then get stuffed through small holes in the bottom layer, before a further final layer is added to cover those holes.
I hope this helps and is legible.