If I understood your question - you can declare the achievements in whichever rpy file in your project you like. The included achievements.rpy file has several example achievements, so it probably makes sense to declare your own achievements there. Be sure to read the included README for more instructions on what details you can provide to each achievement as well! You might consider reading https://feniksdev.com/organizing-a-renpy-project/ if you're new to Ren'Py project organization in general.