This is a piece I made a while back (2016 in fact) and is meant more as a build log as opposed to an instructable but I'll do my best in explaining my process and answer any questions any of you have! This is a sculpt I made using air-dry clay of Mr. Spock and represents a tribute in memory of Leonard Nimoy. This was my first time sculpting something of this form and size and I closely followed the video below:

https://www.youtube.com/playlist?list=PLrVj468UL3z...

In any case, here's a rough log of how I went about doing it!

Supplies

PVC Pipe (depends on the shape and size of your bust sculpture)
Foam (or any light thick material to use for initial shaping of the head)

Duct Tape

Air-Dry Clay (other clay may be used depending on your purposes, more into this later)

Sculpting Tools (you can make your own as shown here! )

Step 1: Build Up Your Base

Get your pipe and foam and make a rough base for what your sculpt will be. I drilled out a hole in a piece of plywood and stuck the pipe in to hold it up. I unfortunately don't have any pictures of this process so you may refer to the Youtube playlist above. Just imagine a pipe with foam duct-taped to it. If you're making a piece of modern art, you may call it done and stop here. After I had a rough estimate what my head was going to be, I started smoothing out the shape with the clay to the point where I had a loose shape of a head.

Step 2: Here Comes the Face

This is when I started defining the face.

I first began with getting the prominent features of the face done, this included the cheek bones, nose, lips, and brow (we'll see the eyes next step). Before any large pieces of clay can be added however, I had to rake the surface where the piece was going to go and used slip (clay + water) to function as a glue between the two pieces to strengthen the bond. Don't try to sculpt everything perfectly first time, you have to think very geometrically with the facial structure. This can be seen with the nose in particular. The bridge of the nose is essentially a rectangular wedge shape with two balls on either side. These make up the nostrils. I smoothed all the shapes together until it was to my liking and then took my sculpting tool and carved out the actual holes for the nostrils. After refining the main facial features, I went back in with pieces of clay to fill out the cheeks so the face didn't looks as hollow.

It's worth noting to not put so much foam at your base that it'll poke through later. You can see where I had this problem in the left eye socket. This was fixed later when the eye was added but it made it difficult in trying to shape the initial face.

Step 3: I See You

The eyes were initially large balls of clay stuffed into the sockets. There are two processes that can be taken to make the the eyes from here. You can make thin sheets of clay to create the eyelids or you can carve the center of the eye out to recess and make the eyelids. The latter is a bit more of a complicated process but this is what I did. Initially the eyes were a bit too wide for my liking, that in addition with the raised brow and puckered lips, he appeared rather sad. I believe I did end up layering some sheets of clay for the eyelids to help the overall eye-shape afterward as well as bring down the brow and do many trials and errors for the lips.

Step 4: Are You Listening?

Look at many pictures of ears for reference.

The ears were definitely a point of struggle with this project. The ears were made with a sheet of clay along with several snakes of clay. Attach the ears after raking and slipping the attachment points. I also made snakes of clay and made slight grooves in it for the eyebrows.
In retrospect, they were probably made a little too large but what you gonna do 4 years after the fact?

Step 5: The Hair

At this point, the bust was becoming too dry so when I added the hair, because the clay shrinks, the hair would crack around the head because there was no give to shrink into. If this happens to you, wet the clay and allow the water to soak in before adding more clay. The hair was flopped as one big sheet and cut to style once on his head. It then had light grooves sculpted in just to help detail it as hair.

Step 6: The Pedestal

After the head was finished, I sculpted a pedestal. The ring midway around the base broke off after shrinking and this bit had to be scrapped later on. After all this had dried, I made a circular base from wood, drilled a hole in it the size of the PVC pipe, stained and varnished the wood, carefully pulled off the sculpture and sat it into the new circular base.

Step 7: Painting

After all the 3D work was finished, I moved on to the quickest step. Painting. I started off by spray painting the overall piece white and after it had dried, I took a sponge and lightly dabbed on some light gray in the recesses and around the piece to give it more of a marbled finish.

Step 8: Finished!

Step 9: Notes

If you chose air-dry clay, try to keep it moist for as long as you can while you're working on it. When you're not working on it, cover it in a plastic bag to keep it from drying out.

If your clay dries out too early, wet it with some water and allow it to soak in. Don't drench the piece. Use additional slip and raking to help add more pieces.

If pieces break off because of shrinkage and is too dry, you may need to glue it back on. (I had to do this in places with the hair)

If you want to sculpt this as the first step in a mold-making process, you can choose Monster Clay which doesn't dry out and you can reuse it for other sculpts after you've made your mold. To soften this clay, heat it with a heat gun, crock pot, or microwave.