A clean plastic grocery bag
Duct tape (any size, I just cut or tear it into thinner strips)
Cotton Balls (you don't need a ton, but hey, they have other uses, so you might as well get a big bag!)
Small pair of household scissors
Even smaller pair of manicure scissors
A Doll (I'm using an MNF, the size bag you use will vary according to doll size, MNF is just a standard sized one)
You probably won't need to unstring your doll depending on the make, but if you are too afraid to do so if the head doesn't come off via a clever little system like Luts uses, don't worry!! - you can still follow this tutorial, just cut a hole to go over the head, then wrap a little bit of left over plastic around the neck.
I didn't get pictures of the first few steps (sorry!
) but the general jist of it is this: Cut your grocery bag in half. Drape it over the doll, and pull tight around the body. It doesn't have to be perfect, this isn't a dress form for making cat suits, more like t-shirts. 
You will start from the bottom, hold the legs close together and bind them with tape. Wind upwards over the body like you would a mummy, using long pieces (about as long as your forearm) pieces of duct tape. Once you get to the section under the bust, stop, and win a piece of tape around the neck, then cross it over what would be the collar bone, as shown:
http://farm4.static.flickr.com/3248/...1f15f5c9_o.jpg
Once you've got that down, tape over the bust, then tape up the arms. Don't worry, the arms should slip right off, and if they don't, you can always cut slits in them later to get them over your doll's hands.
Once everything is taped up, it's time to start cutting! I do NOT recommend using a razor or ex-acto knife for cutting. You might cut into your doll's resin, or yourself, and neither are going to end in anything but your tears. Pick up a small pair of scissors, and starting at the base of your duct tape form in the back, start to cut upwards.
http://farm4.static.flickr.com/3106/...f47cf185_o.jpg
You can cut about halfway up the back before you need to switch to the manicure scissors. Scissors allow you to point the blades away from your doll, so you don't scratch it.
http://farm4.static.flickr.com/3223/...8efb69d7_o.jpg
Once you've cut a straight line from the bottom to the top, you can peel the plastic off of your doll. I generally get it mostly off the shoulders, then point my doll's arms forward, to get the duct tape "shell" off with a minimum of fuss.
http://farm3.static.flickr.com/2388/...5067c67a_o.jpg
...I'm totally kidding here, because man, crayfish are TASTY. Anyway!
Once you have your doll free of the dress form, go ahead and set it aside, put some clothes (and the head, if necessary!) back on your doll, then put it away. You won't need it from here on out. I begin to tape up the seam in the back from the middle, because I can place my finger underneath it so I don't crush the form, and so I can make sure that the seams will line up the rest of the way properly.
http://farm4.static.flickr.com/3085/...9489c680_o.jpg
Once you've taped everything up, it will look like this:
http://farm4.static.flickr.com/3227/...06a1e54b_o.jpg
Since there's nothing holding it up but air, the form will look a little crumpled. Not to worry though, we're going to fix that right up.
http://farm4.static.flickr.com/3118/...e2fd9ccb_o.jpg
Once you've stuffed the neck pretty tight with cotton balls, go ahead and tape it off. Then you can start to jam cotton balls into the rest of the form, starting with the bust, then down through the middle, and in the case of MNFs, the huge butt before you finally make your way down to the legs. Once those are all nice and packed (you want the form pretty stiff!), go ahead and tape up the bottom.
http://farm4.static.flickr.com/3126/...aa1db567_o.jpg
Finally, you'll tape up the arm holes, and then...Ta da! Completed dress form. If you want it to stand on its own, a dowel rod with a bit of glue on it shoved up in the midst of all the not entirely densely packed cotton will fix that. Once dry, you can pack in more cotton, and then set the rod in a base.
It's not pretty, but it is a quick and easy way to get a dress form that you can shove pins into when designing clothing for your doll. I hope you find it useful! Aaaaaand I hope my hand writing wasn't TOO messy.
Tutorials