To start the process you put on some clothes that you do not mind destroying, and then have somebody wrap you in duct tape from neck to ankles. You really want somebody patient who you really trust because this will take quite a while, and you will be completely incapacitated by the end of it. In retrospect, my wife was probably not the best choice...
It is probably a good idea not to wrap the duct tape too tightly. In addition to being really uncomfortable, it can make very difficult and possibly painful to cut yourself out of the duct tape. I learned this the hard way. Scissors did not work very well for me in cutting through the duct tape, but tin snips did fairly well. The wife was kind enough to help cut me out of the duct tape prison, but not quite kind enough to do so without cutting me multiple times. Once she got one of my arms free, I figured I would bleed less if I did the rest myself.
In her defense, it was really difficult to avoid cutting myself in the process ( I may have bled some from my own efforts as well). Just to reiterate - don't wrap the duct tape too tight, you will probably cut yourself trying to get out of it. Eventually I got the whole suit removed.
Once you have escaped the duct tape suit, you need to build a frame to go inside it so that it can stand up. I would recommend one inch PVC pipe because it is cheap, easy to cut, and all the connectors you need are available at your local hardware store. By the way, it is a lot more convenient to cut if you pick up one of these:
I forgot to take a picture of the PVC frame after I assembled it, so you will have to make do with my terrible MS Paint representation. You will need five T joints and two ninety degree elbow joints to make the frame, or seven T joints if you want to make the arms detachable ( I only did this with one of the arms).
I used mostly balled up wrapping paper and grocery store bags to fill the suit. After I built my suit I realized two things that were going to be a pain when using it. One was that I made the hips too wide on the frame and it stretched the torso sideways. This not only looked weird, but would make my measurements off when I was using the mannequin. This was a fairly simple fix, I just cut it back open, pulled apart the frame, and shortened the hip section. The second problem was that the arms were too stiff for me to easily put on and remove shirts and jackets and such. To resolve this I cut off the left arm, switched out the elbow joint for a T joint, put a short section of pipe into the T joint, and added an elbow joint inside the arm to reattach it.
This allowed me to remove the arm when I needed to, as well as swivel it when it was on the mannequin. That is pretty much it on the build. All you need is time, a ton of duct tape, some PVC, some padding, and a patient friend.