Here is a quick step-by-step reference for bust-making as I know it, with a slide show of the basic process at the end.
Step one- Find a model who is willing to get covered in gloppy gross stuff and encased in plaster while sitting uncomfortably for several hours.
I give you, Brianna!
Step 2- Discuss the pose with the model. Is it something they can hold in place for 2 or more hours, even when they are slippery with petroleum jelly? Does the pose accomplish what you want artistically? Is it remotely comfortable at all? Etc.
Tip- Do not plaster a clausterphobic model. They will freak the heck out. If the model doesn't realize they are clausterphobic until the middle of this process, take the cast off immediately, even if it means utterly detroying it. Crap happens.
Tip 2- Assess the pose: can you get the plaster cast off your model, or are there undercuts you'll have to avoid? Will you have to do different sections, or can it be all one piece? Remember, the plaster will be hard as a rock when it sets. Imagine trying to take off a sweater that went completely rigid. . . Know how you need to plaster the model so that it will come off easily.
Step 3- Make sure you have all the necessary supplies.
Supplies: plaster gauze cut into rectangles (mostly large, some small for detailed areas), very large bowl or bucket of luke-warm water, petroleum jelly, paper towels, access to a shower that you might have to clean later, shower cap, a trash can with the lid off, tape, plastic sheeting, and somewhere cushy to lay the finished cast. I'm assuming that there are things like soap, towels, etc. accompanying the shower. And ask the model to bring a change of clothes.
Step 4- Cover the plastering area (table, chair, floor, anything you don't want to scrub plaster off of later) with plastic sheeting. Any kind will do the trick, just tape it down to whatever it's covering so it doesn't slide around during the process.
Step 5- I ask models to wear an old sportsbra or bathing suit or something. Depends on the final piece you're aiming for and the comfort level of the model. BUT, the more clothes they're able to wear, the less petroleum jelly and plaster drips they'll have to scrub off in the shower later. And it's nice to not have to scrub tender places.
Step 6- Cover the model's hair with a shower cap, and make sure you have all the little hairs tucked up under the cap. Plaster will pull out any hair it can, and your model won't thank you for that! If your model is a really hairy guy, be extra super generous with the petroleum jelly in step 7.
Step 7, Slather the model in petroleum jelly (they usually do most of the slathering, but occassionally need help, especially after they start getting all slippery). Try not to have lumps and clumps of petroleum jelly; just a nice, thick, even coat. Again, hairy guy= put on a REALLY thick coat.
Step 8- Cover the model's eyebrows and eyelashes with small wet paper towel pieces. Yep, the plaster pulls out those hairs, too, so cover them up really well.
Step 9- Ask the model to get into the pose, and adjust as necessary. Now it's time to work as quickly as possible. The faster you get the rest of the steps done, the easier it is on the model. Your pre-cut plaster strips and luke-warm water are on the table ready to go, right? Lets begin!
Step 10- IT'S PLASTER TIME! Dip a plaster strip into the water, slide it between your fingers to remove the excess water, place on the model, and smooth. Do this repeatedly until you have covered everywhere that needs it in one (semi-) even coat. Then immediately start on coat #2, following the same procedure. Then immediately do coat #3. I never do less than 3 coats (for structural stability) but if your model is doing fine and you can get a 4th coat on, go for it. Tip- An extra layer around the edges of the cast is very helpful in keeping the edges from crumbling and being weak. LEAVE NOSTRILS CLEAR AND UNBLOCKED AND GENERALLY ABLE TO FULLY FUNCTION. This is art, not homicide. Please do not suffocate the model.
Step 11- Make sure the cast is dry before you take it off. It will start to look more solid and less wet, and if you press on it lightly, it won't flex. When it has hardened, gently pull it off the model (do this slowly so the model can pull themselves out of it as you go and get out of the pose without breaking the cast). Lay the cast on that cushy surface you set up earlier. (I hope it's not your mom's nice couch cushions . . .)
Step 11- First, human clean up! Give the model a bunch of paper towels so they can wipe off as much remaining petroleum jelly and plaster bits as they can (I throw these straight into the trash can so there's less mess) and show the model the way to the shower. Try to keep from touching anything on the way there, or there will be slimey stuff all over it. The model should have access to liberal amounts of soap/shampoo/something like that. Also, hopefully he/she brought a change of clothes.
Step 12- Work-space clean up! I carefully pick up the plastic sheeting (keeping the mess inside) and throw it away. Yes, this is a one-time use of something that is normally used many times. Yes, I have tried re-using the plastic sheeting and it is a nightmare. Throw it away, unless you get a very great deal of enjoyment out of cleaning. Set the plastery-water aside for the night and let it settle; the next day, you can safely pour the water off the top of the bucket, and the sludge in the bottom can go in the trashcan. DO NOT POUR PLASTER-WATER DOWN ANY DRAIN EVER. Plumbers are expensive.
Step 13- If there is any other messiness that needs wiping up/scrubbing/throwing away now is the time. It's very unpleasant to sit down to breakfast the next day and find a slurp of petroleum jelly with your elbow. (I do indeed plaster cast at my kitchen table- it has ready access to a shower and is open for art-making 24/7).
Last bit- Let the cast sit for several days before you use it, and wipe out any excess petroleum from the inside that you can. If you can let it sit for a week to dry, even better.