Obtain a hard, powdery bar of soap. The best bars for carving are ones that produce a fine powder when scraped, rather than being soft like putty or flaking off in large chunks. Look for a bar that's thick and doesn't have a curved back, as you'll have better luck getting what you want out of a regular shape.
Scrape away the soap's distinctive marks. Carve down any decorative edge grooves. Use a knife with a long blade to shave down any sides with writing or imprinted symbols; cut down the entire side until the imprinting is no longer visible.
Outline the shape of the object you want on each side. Look at each side of the soap bar in turn, and try to imagine what your finished object would look like from that angle if viewed as a two-dimensional outline. Carve this outline in the bar. Check your outlines on each side as you go and judge them against one another in terms of height, size and positioning to make sure they line up.
Gradually chip away the soap outside the outlines. This will leave a rough, three-dimensional shape, as if the finished sculpture were dipped in a thick coating of wax. Imagine the sculpture in this way to help envision how you will continue reducing it into the shape you want.
Refine the shape of the carving. Again, carve lines to indicate the shapes you want and gradually whittle them down into the form you want. Use a craft knife with a small, pointed tip to help you reach small crevices. Remove dust from your carving as you go using the blush brush rather than blowing air from your mouth; this will keep the soap from getting moist.