You will find many feather duster tutorials online. I think mine is much simpler than most.
Buy at least two cheap feather dusters. I found mine at Big Lots several years ago for 69 cents each. You will not be able to re-use them...you'll be lucky to complete the job using only one.
I like to use a paint that is just barely darker than the rolled wall color. In this case, I am using Behr White Peach (Satin) as the base wall color, and Glidden Chemise Pink (Flat) for the feather painting. This gives a very subtle effect, and looks rather like damask fabric.
To paint my room, which is approximately 13' x 15' x 8' high, I used approximately 1 cup of paint. I started with a quart of paint, and still have most of it!
Keep a damp cloth handy.
Pour the paint into a paint tray. Enough to cover the bottom of the tray well about 1/2".
Dip the tip of the feather duster into the paint. Blot it a time or two on the flat part of the tray.
You might want to practice on a piece of cardboard, first. If you're confident, you still might want to start in the least noticeable corner of the room, or where a large piece of furniture will conceal your first effort.
Once you start, try to keep going.
When using a color similar to the wall color, you will need very good light to see where you have painted. I used the room's overhead light as well as a floor lamp (shade removed) with 3 100 watt bulbs. I moved the lamp as I progressed around the room.
Apply the paint to the wall, holding the feather duster straight in front of you. This method is something like sponge painting, but much easier and far less messy.
Dab the paint on the wall. Dab, remove, turn the duster, dab again. Turning the duster will keep you from having the same imprint over and over. I can usually make about 10-15 imprints before I need to load the duster again.
This was difficult to photograph...it is not really this dark!
Work quickly, without over-thinking the process. The more pressure you apply, the larger and mushier the imprint will be. If you make an imprint you really don't like, wipe it off immediately with a damp cloth (assuming you're not painting over flat paint).
You will also want to keep your damp cloth handy to remove any imprints from the trim.
You move very quickly with this method. I was able to feather paint this room in just over an hour.
When you finish, check to see if you missed any large areas.
Because you're using very little paint on the duster, this is a very tidy process. I don't use a drop cloth, but you might want to. Because you have the nice long handle on the feather duster, you'll get very little paint on your hands.
Next.... painting lattice for the border, using a stencil.... stay tuned...