Complex sentence: In May 1944 before D Day Stevenson was posted, as a CSM, to the 1st Buckinghamshire Battalion (1st Bucks) of the Ox and Bucks landing on D Day, 6 June 1944, near Ouistreham.
Split into simpler sentences: In May 1944 Stevenson was posted, as a CSM, to the 1st Buckinghamshire Battalion (1st Bucks) of the Ox and Bucks. On D Day, 6 June 1944, he landed with 1st Bucks, near Ouistreham.