Hi there,
I bought this book on free diving training. It is very detailed but a long boring read haha. So far I am just trying to apply it.
a TL;DR version is maybe this article from FreediveUK. Lacks detail but give you an overview of the training.
I am using this mostly to train on land until I can hit the water again.
The CO2 and O2 tables have helped me greatly (an overview on it here). But manually timing myself, etc, was such a pain. I bought the app STAmina, it is the easiest and most intuitive of all the apps I tested. It will auto generate your tables based on your personal best time. It allows you to focus on your training instead of fiddling with a lot of settings.
hope it helps