Whatever you do, don't go cold turkey. Both of the drugs you mentioned are dangerous to taper off of suddenly. Both cause extreme blood pressure spikes after tapering.
If your doctor is being stone-walled and not accepting your plea's for help, and you are absolutely sure you want to go ahead with this, then I suggest doing a linear reduction of the dose.
I have to admit, I can't find much studies on tapering for ADHD meds, and the textbooks I have don't state anything, but I could find lots on SSRI tapering. Since it's well known that receptors in the brain adjust linearly to a dose of medication, then I don't see why SSRI tapering strategies wouldn't work on ADHD meds.
A good source I have is "How to taper off your SSRI" on the Harvard medical website.
The general strategy with clinicians is to slowly reduce the dose by either 1's for guafacine and 10's or 5's for methyl depending on your tolerance. Wait 5 half-lifes (1) to check for any symptoms just to be safe. Then, reduce again. Repeat until fully off.
(1) The half life is how much it takes for half of the drug to go into your urine or feces.
