First you set your python path on RIDE's Tools->Preferences->Importing Pythonpath so it does include your library and all its dependencies (imported modules and so on), like this:

(If your library is distributed via PIP, just pip install my_library_package_name instead, on the same environment you are running Robotframework/RIDE)
Then you can add the library to your test suite on the add library dialog you can open by pressing the library button on the Edit tab you can see when you select the suite:

After that, all keywords defined in your library will be available for use in any test of the test suite. You will see a Library import sentence on the Settings section of the header of your suite's code (On your text editor or RIDE's Text Edit tab of the suite:

Sometimes RIDE doesn't recognize the keywords on a recently added or modified library right away. In that case, Save all (CTRL+SHIFT+S) and restart RIDE to fix the issue.
Also, watch out for the library entry showing up in red on the suite import list on RIDE's Edit tab; it means something went wrong when trying to import the library. If you need to know what, you can find the trace on RIDE's Tools->View Ride Log.