Say a word in the lowest tone you can possibly give off. Repeat this word 10 times, focusing on getting the raspy tone you are looking for and on how it feels to your throat. Remember the feeling you get in your throat when you get the desired raspy tone so you have an idea of how your throat should feel when you do raspy vocals.
Read a paragraph using the deep raspy voice you developed and repeat it until your throat gets used to pronouncing the words in the low and raspy voice. Practice this on a daily basis to get your throat used to this voice. Practice is essential.
Sing loudly, to the point where you are shouting. Sing songs from bands that have raspy singers as a guideline. For instance, Rage Against the Machine, Nirvana, Bob Dylan, Allman Brothers and Tom Cohen use raspy vocals. This type of exercise will strain your vocal chords and give you a raspy voice after singing for an extended period of time. Have a hot nonlactose drink with you when performing this exercise to soothe and warm up your throat.
Say the word "Ahhh" using minimal air. This literally means that you should be using as little air as possible. Singers fill the diaphragm with air to sing and hit extended notes; however, when you refrain from using your diaphragm, your vocals are forced to come out from your throat rather than from the air stored in your diaphragm. Consequently, the strain in your vocal cords gives you a raspy sound. This is a good way to incorporate raspy vocals into your singing because you can choose when to sing from your diaphragm and when to sing from your throat.