Build a Spaced Repetition Study System Without Any App

Spaced repetition works when the review schedule is maintained; it collapses when the system is too complex to sustain. App-based systems add a layer of friction between idea and card, and many people abandon them when the review queue grows beyond manageable. This builds the same core mechanism with index cards or a spreadsheet, on a review schedule designed for real weeks rather than ideal ones.

I want to use spaced repetition to retain what I am learning but I do not want to rely on Anki or any other app. Help me build a paper or minimal digital system that uses the core mechanism without the overhead. Cover: how spaced repetition actually works and what the evidence says about review intervals, how to create a simple card system using only index cards or a spreadsheet, a review schedule that works with an imperfect week rather than a perfect one, and the two common mistakes that make self-built spaced repetition systems collapse after a few weeks. I am studying [subject or type of material].