Complex sentence: In December 1985, Chauk Pa-daung Sunlun Monastery, a branch of Kaba-Aye Monastery was established in Upper Burma though they had already formed a Sunlun Meditation Group since 1977 there and had practised the method regularly with Sayadaw leading the December Retreats annually.
Split into simpler sentences: In December 1985, Chauk Pa-daung Sunlun Monastery, a branch of Kaba-Aye Monastery was established in Upper Burma. A Sunlun Meditation Group was already in operation there since 1977, practicing the method regularly, with Sayadaw leading the December Retreats annually.