Pipe joint compound is available in a tube or in a canister with a brush applicator. The manufacturer specifications are an important consideration when purchasing a sealant for your particular project. It is important to note if the product is designed to be used on a certain type of pipe or if it can be used on both metal and plastic. When buying pipe joint compound I usually look for a product that can be used on all types of plastic and metal threaded pipe and keep this in my toolbox among other basic plumbing tools.
To use pipe joint compound simply apply an even coating onto the end of a threaded pipe using either the applicator brush or a finger. Then thread the pipe into the fitting making sure to avoid cross threading. I often use pipe joint compound on top of Teflon tape for a great seal on almost any threaded joints involving water.
Note: One important thing to keep in mind is that sealants are not always appropriate for both water and gas. Teflon tape for example comes in two types: one for use specifically on gas and one for water pipe. This can be true for pipe joint compound also, so read the product usage specifications carefully.