We haven't adopted (yet!), but as I understand it, you simply apply.
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http://www.servicecanada.gc.ca/eng/goc/cctb.shtml
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If someone else is already receiving the CCTB for your granddaughter, Service Canada will need some extra documentation to switch it over to you as her primary caregiver. In my experience, the best thing to do is start an application, then call them for help or further clarification - so that way they are looking at an actual file on the screen.
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Good luck!