Glad you asked...
For serious addicts that inject their drugs, they find that the high is not as strong as they thought it would be. They crush their pills and let them dissolve in distilled water, with some heat applied. They then filter that through their choice of material, and then filter it once again. Because of them being pills there is a great deal of fillers used that make it unsuitable for IV injection. On the final filter the draw the liquid up into a needle ready for injection.
Then again, this answer might help you...
Adderall is never recommended to be injected. It is illegal to take without a prescription and shooting up, even a diluted pill, can have serious health effects.

