When piece measures 1920 cm adjust so that last row is from RS then work in GARTER ST see explanation above until finished measurements AT THE SAME TIME dec every 4th row as follows

Can you repeat this sentence, but add in punctuation?
When piece measures 19-20 cm (adjust so that last row is from RS), then work in GARTER ST - see explanation above - until finished measurements AT THE SAME TIME dec every 4th row as follows: