First of all, speaking as someone with bipolar and GAD who struggles with meth addiction: methamphetamine abuse and mental illness do NOT mix. Even if the elevated risk of a psychotic episode doesn't faze you, please understand that the extreme highs and lows of this substance, however fun they can be, are incompatible with the mental stability and clarity you need to be healthy. And anecdotally speaking, a good friend of mine with severe ADHD once took a single hit from my pipe and hated it, feeling uncomfortably overstimulated and sleepless for a long time after.
Have you tried seeing a doctor or psychiatrist first? If you do have diagnosable ADHD, the doc can prescribe a med that might work better for you than meth, with the added benefits of being legal and supervised by a professional, so if the med affects you too strongly or is ineffective, you and your doc can work together to find one that works. Also, though it's popularly associated with depression and anxiety: therapy, especially Cognitive or Dialectical Behavior Therapy, is really the key to taking back control of your thoughts and mind.
That said--if you're dead-set on self-medicating, use the prescribing practices for Desoxyn (prescription methamphetamine for severe ADHD) as a guide. Desoxyn comes as an orally-administered 5mg tablet, and according to one medication-info website, the usual therapeutic dose is 20 to 25 mg split in two daily doses (presumably morning and afternoon). If you really want to use meth as a medication, you MUST use sub-recreational doses. Start with just a tiny 5mg shard, only orally, and titrate upwards as needed, just until you feel some relief of your symptoms.
This may work as a temporary fix, but even if you exhibit total self-control in dosage, there are too many variables, from the quality/purity of the shard to tolerance and side effects, to make self-medicating a viable path to relief from ADHD symptoms. If you really want to fight this disease, and not just get high, you're going to need to work at it. It sucks, but you can't expect a drug to magically fix your brain; YOU must take an active role in controlling your thoughts and actions in order for any treatment to be truly sustainable.