To get the Disability Tax Credit you complete the identification portion on the first page of the Canada Revenue Agency's form. If your income is lower, you should indicate who the Credit can be transferred to - often a spouse, parent or sibling that is providing care. You should also tick the box to have previous Income Tax Returns reviewed and adjusted, as the Credit can be backdated.
The form then goes to a medical practitioner (may not necessarily be a doctor - check the list at CRA). They complete the form, detailing the extent of the disability, and how it interferes with common, daily living, including the date of the onset of the condition.
Once signed by the medical practitioner, the form is sent to CRA (it can be scanned and sent electronically with CRA MyAccount). CRA reviews the application, and decides if the disability meets their criteria. This may take up to 6 months for a decision.