The first thing to do is to initiate an Ionic app.
This is pretty straight forward using the "starters" and, if you are not familiar with this, the best way to begin with is reading the Ionic documentation:
Once the environment is ready, it has to be connected to your Firebase project.
There are multiple guides; this one for example focuses on Ionic 5 and Angular.
If you try to launch the app after you follow the guide, you will probably end-up with the error:
ERROR in node_modules/@angular/fire/angularfire2.d.ts:37:49 - error TS2344: Type 'T[K]' does not satisfy the constraint '(...args: any) => any'.
To fix this, you will have to edit your tsconfig.json file as per this question on stackoverflow.