First, you have to file their final return. This includes all income received up until the date of their death. If the date of death is before October 31, the return is due by April 30 of the next year; after October 31, it is due within six months of the date of death.
Second, you must file a T3 Trust Return. This includes all income earned by the estate from the date of death until the estate is wound up.
Finally, when the Assessment Notice is received for the final tax return filed, a request should be made to Revenue Canada for a Clearance Certificate. The Clearance Certificate will confirm that there are no further taxes owing by the deceased or their estate and any funds remaining in the estate can be given to the beneficiaries.
Estate taxes can be difficult and complex, and you can be liable for misfiling if you are the executor. This is a situation where it may not be appropriate to do it yourself; an accountant can help immeasurably with estate taxes.