心の儘 is the heart just as it is. 心 is the heart and the mind together, and 儘 is the state of a thing as it stands. So the phrase names the condition of doing what one's heart directs. The dictionaries give it as just as one thinks, at one's own discretion, to one's heart's content. Freedom is its sense. Follow your heart has the sense right; the Japanese describes the state where the English gives the instruction. 儘 is the full form of the character, and the word is also written in kana or with 侭, all one word.
心の儘 is composed of the kanji 心 (read kokoro) meaning "heart", the possessive grammatical element の (read no) and 儘 (read mama) meaning "as it is; as one likes".
心の儘 is a Heian phrase, as the heart wills, in the 竹取物語 of about 900 and in the 源氏物語, where it can already carry license as well as freedom, doing as one pleases.
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 心の儘.