How do I amend my past taxes to include deprecation on a rental property?

How far back do you need to amend? This matters. If you did not depreciate your rental for two or more years, or if you took incorrect depreciation for 2 or more years, then amending is not an option. Instead, you have to file IRS Form 3115 to correct the depreciation.
The form 3115 is not simple by any stretch, and for that you should seek professional help. Especially if your state taxes personal income.

See IRS publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf starting on page 13 at the bottom. One thing it states is that you generally must file form 3115 if:

• A change from an impermissible method of determining depreciation for depreciable property if the impermissible method was used in two or more consecutively filed tax returns.
• A change in the treatment of an asset from nondepreciable to depreciable or vice versa.
• A change in the depreciation method, period of recovery, or convention of a depreciable asset.

There is more I haven't listed here.