There is this image that Hollywood loves to use. It depicts the magical practitioner engaged in some dastardly deed via sympathetic magic using a figure (mistakenly called a voodoo doll) and doing something like pricking it with pins. The term poppet is actually more accurate. These have been in use in folk magic from just about every culture since antiquity. The term poppet comes from the English culture. It is still used to refer to dolls. It is also used as a term of endearment for young children and women.
The poppet in magical use is akin to an icon for a deity. It serves as a magical focus that assists the practitioner in directing their spell to the correct target. They are constructed in many different ways. In some cases, they are used from things in nature that are approximately humanoid in shape (such as mandrake root). Most often, modern poppets are made from fabric. They are stuffed with any manner of herbs and filler material, much like the cloth poppets of antiquity. There is a wider array of herbs and other components that can be placed inside a poppet in the modern era because of the fact that we're no longer dependent upon locally sourced materials (for the most part).
Traditionally, a poppet included some physical tie to the target. This was usually hair or nail clippings. Now, it is possible to use anything from a picture of the target to an item that strongly reminds the caster of the target. The use of nail clippings have fallen out of favor due to the general concept that it is unhygienic to handle them. Hair clippings are still used on occasion but they have become even harder to acquire with the fact that people go to hairstylists for their haircuts as a general rule.
This makes it harder to isolate the target's hair and puts the would be caster in the position of having to somehow take a hair from their target's person. Given the laws against assault, and the fact that cutting another person's hair with out their permission is considered to be an assault on their person, it is inadvisable to go this route. Hair taken from a willing target is, of course, easily obtained. In many cases, however, the target of the spell is not easily accessible and the caster needs to use an alternative method to 'tie' the poppet to their target.
Once the poppet is 'tied' to the target (spiritually resonant with them), it is manipulated in the manner that reflects what the caster is attempting to enact. In some cases, pins are inserted with the focus of causing pain. In other cases pins are inserted like spiritual acupuncture. They can be tied, burned, buried, or have any other manner of things done to them. Because of sympathetic magic, there should be a related effect upon the target. Once the poppet is no longer useful, the practitioner will sever the magical tie between it and the target it represents. Then, the poppet is destroyed.
In some cases, poppets are not destroyed but rather rendered magically inert to be recycled for future use. This is done with poppets that do not have physical links to the target embedded within them. Those that have physical links to the target of the spell should be rendered magically inert and then destroyed, unless the caster has future spells they wish to perform upon the same target. The reason why these poppets need to be destroyed is because the physical ties to the target will continue the resonance, albeit at a less effective level. This complicates future recycling of the poppet and makes it harder to attune the poppet with a new target.