Bhai Kahn Singh Nabha's Mahan Kosh contains the following note about Sukhmana Sahib:
ਪੁਰਾਣੇ ਸੱਜਨ ਗੁਰੁਬਾਣੀ ਦੇ ਪ੍ਰੇਮੀ ਸ਼੍ਰੀ ਗੁਰੂ ਰਾਮਦਾਸ ਸਾਹਿਬ ਦੀ ਚੌਵੀਹ ਅਸਟਪਦੀਆਂ ਨੂੰ "ਸੁਖਮਨਾ" ਮੰਨਦੇ ਹਨ. ਛੀ ਅਸਟਪਦੀਆਂ ਬਿਲਾਵਲ ਦੀਆਂ, ਛੀ ਸਾਰੰਗ ਦੀਆਂ, ਛੀ ਕਾਨੜੇ ਦੀਆਂ ਅਤੇ ਛੀ ਕਲਿਆਨ ਦੀਆਂ।
As per the above note, Sukhmana Sahib contains Siri Guru Raamdaas jee's 6 Ashtpadis of Raag Bilaval, 6 of Raag Sarang, 6 of Raag Kanra and finally 6 Ashtpadis of Raag Kaliyaan but as far as I know, Sukhmana Sahib contains 6 Ashtpadis of Raag Nat-Narayan as opposed to Raag Sarang. Raag Sarang does not have any Ashtpadi of Siri Guru Raamdaas jee Maharaj. It only has Ashtpadis of Siri Guru Nanak Dev jee Maharaj, Siri Guru Amardaas jee Maharaj and Siri Guru Arjun Dev jee Maharaj.
The subject matter (Visha-Vastu) of these Ashtpadis is extreme Bairag for Darshan and chanting Naam. It also contains great statements of hope that enable the fickle minded to become strong and chant the true Naam. The rhyming of all these Ashtpadis is Ascharaj-roop (wondrous, amazing). It seems that even though these 24 Ashtpadis are spread across in 4 different Raags but they were uttered by Guru Sahib at the same time.
Dream Kirtan Darbar: Dream Kirtan Darbar is to do Sangati Siri Asa kee Vaar Kirtan along with Siri Sukhmana Sahib. Each of the Ashtpadi of Siri Sukhmana Sahib should be sung with each of the Chhaka of Siri Asa kee Vaar and it should be sung "Vigas Vigas ke".
Kulbir Singh