Lime Sulphur is usually used in the Horticulture industry to control scale insects, moss, lichen and fungal diseases on plants. It can be used for all of these issues when it comes to Bonsai. A use that is not listed on the container is to whiten deadwood / Jin.
As always, safety comes first when you use any chemical. In this case, it is not just the smell (rotten eggs comes to mind), but also the fact that this chemical is corrosive. It is a good idea to wear gloves and to protect your eyes. Wash your hands very well afterwards and keep your hands away from your face.
Follow the instructions on the container for all uses. As said, the whitening of deadwood is not listed as a normal use of lime sulphur. One thing to keep in mind is to also protect the soil surface from lime sulphur dripping or being spilt on the surface. Remember, this chemical is also used to kill moss! You do not want to kill of your lush green carpet covering the soil surface. I usually use plastic wrap for this purpose.
I dilute the lime sulphur 50/50 with water and I do this in a plastic or paper cup as it is easier to just throw it away afterwards than trying to clean it and getting rid of the smell. I also use a 12mm brush for most applications, but can go smaller or larger depending on the size of the job.
I also find it easier to apply if I slightly wet the surface of the deadwood. The Juniper that I worked on here had a bit of rot at the bottom where the tree meets the soil surface. This was cleaned well and the lime sulphur was liberally applied here. The chemical protects the wood against rot by acting as a preservative.
After application, clean the brushes well and get rid of the container. Wash your hands and allow the lime sulphur to dry and work its magic on the tree.