Echoing the others: don't.
It can be done, and I have done it – both feather and artificial.
With feather, you need an excellent drying day (ideally warm and windy), and you have to go out and clap it and move the feathers around frequently. If it's feather, it will also weigh so much wet that the seams or fabric may rip if you lift it wrong.
If you're determined to go ahead, try the following.
Warm soapy water in bath (special cleaner if it's feathers).
Get in and tread it, although the detergent may not be very nice to your feet/legs.
Pull out plug and leave to drain for some time, eventually rolling duvet up to squeeze.
Fill bath with rinsing water and repeat the above. You may want to do this twice.
After the final long drain in the bath, then the rolled up squeeze, you may still have a very heavy duvet. At this point I've found it useful to put a clothes horse flat over the bath and manoeuvre the duvet onto it for further dripping over the bath.
By the time you can get it outside it may still be very heavy and in danger of ripping, so may initially need to be hung doubled up, or spread over the top of a whirligig.
I have vowed never to do this again ever in my life.
It's such hard work.