Migration to AC Tool is quite easy as it allows to export the AC entries on your existing server. So you can simply install it on an instance that already has the rights that you want. This will work regardless if you managed the entries manually or used some other tool/content package before.
See Installation on how to introduce the tool to either on-prem or cloud service AEM instances.
Do an export using groupBasedDump() on JMX interface. The dump will provide you a Yaml export with all AC entries in your system. Save it to a file and remove all entries that should not be managed by AC Tool. For instance you do not want to manage the system groups such as everyone and administrators. For most non-minimal setups it then also makes sense to split up the one dump file in multiple files for better maintainability.
Create a permissions package in source control/maven and add the config file(s) to it along with the install hook to install the rules during package installation.
Install your package on a clean instance and verify your rules. Take a look at the output of the package installation if there occurred any problems.
Now that your rules are working you can take a look at our best practices to restructure the rules using multiple files and folders.