However, to be able to play in a free and leisurely manner, there are several requirements needed to be achieved first.

(The knowledge of playing techniques and music theories are the basic requirements, so I will omit that part and other than that Chén Yù-jiàn wrote as following-- )

No trouble with fingering movement.
Do not try too hard to create any sound, if trying too hard, the sound will be distorted / not sound natural or genuine.
Do not be affected by philistine’s ears… since most of the great arts do not match public taste…. Besides, the great sound is elegant and very subtle, not everyone can understand and appreciate it…
A good melody is valuing its subtlety, light and yet flavored taste, as eating olives. (But not the pickled olives, I guess.)
Practice well and study for a long period of time (at least 10 years) until one day you’ll realized that reaching the refined state is so easy without any efforts.
Observing playing from good players and masters, as many subtle things cannot be conveyed by oral explanations.
Do not think too hard or too much, otherwise you will suffer from the tangled thoughts.
Only when you are in a free and leisurely manner, you’ll find the wonderful state.
How to have a free and leisurely manner? One must have a pure mind, a pure mind without any opportunistic thought.

北宋成玉礀 的琴論提到一個重點就是要從容自然才能進入琴的道境

而從容自然有其背後支持的條件: (音要調準下指正確等基本技法上的知識在此就先不多提)

指法要不為來去所窘
取聲忌用意太過 太過則失真
不要受旁觀俗耳影響 (大凡事之妙處多不合俗 … 夫正音雅淡非俗耳所知也
成說:調子貴淡而有味 如食橄欖 – 我說:但不是醃過的橄欖 )
用工需深遠 也就是說長期的練習與學習才能累積到造次逢原 不藉力也的境界
在學習與練習的過程中要參考諸家擇其善者從之
許多領會不是言語可傳達的因此要多觀摩(當然觀摩也要觀善彈者)
最終就是不要苦意思 苦意思則纏縛 唯自在無礙則有妙趣
怎麼才能自在無礙? 就是要心中不存機心 機心存於胸中則純白不備
彈琴者至於忘機乃能通神明

也就是說不要想說要彈的怎樣才能吸引人
只有在沒有欲望想要表現的時候
那種心情才能自在表現
不過這也是要在至少彈了十年以上並備足了技法樂理等知識才有可能做到從容自在

More about Chén Yù-jiàn's analysis of good sound of Qin playing...

...敦猱聲貴圓靜
外虛中實
如棉裹稱錘
此聲最要取
縱能者十亦失其一二

“A soft appearance yet with a solid and strong core as a hammer wrapped by cotton, that is the best sound. Even good player might not operate it perfectly. “

It is hard to translate exactly but I think it is the most essential part of how to use your energy to play qin.

...至於未至敦而指先下謂之折腰敦
中徽而無力謂之醉敦
此最不佳
有來去無跡謂之藏頭敦 最妙...

“敦” (Dun) means “throwing,” “casting” or “to manage,” “to control,” I interpret it as the way you manage your energy from your body to your fingertips and to play qin.

There are several kinds of “dun” that Chén Yù-jiàn analyses. For example,

折腰敦(Zhé Yao Dun, bend waist or break waist) – I think it means that you are not ready yet, but you have moved your hands and fingers to touch the strings already. This is not good, because your energy will break and not float fluently and smoothly.

醉敦 (Zùe Dun, drunken)—You reached the hui position but with no energy or strength at all. This is the worst.

藏頭敦(Cán Tóu Dun, hide head)—Come and go without a trace. That is the best.

...凡不攻者取聲則不問軟硬
要得指下有無窮之意
非軟則不可取

A Player who is not serious enough will not care about how to control his or her energy. Only by keeping it soft, one can find the infinite joy of qin playing.

My conclusion: Be Soft outside but solid inside, like practicing Taichi (Tài Jí Quán 太極拳).