It should suffice to 'legally' declare/consider someone a Muslim if he/she makes the following declaration:
"I bear witness that there is no god but Allah, and Muhammad (p.b.u.h) is His last and final Messenger. I solemnly vow to submit, surrender and yield to all of Allah's Laws and Commands as given in His last and Final Book, i. e. the Quran. I realize that in the Hereafter, I shall surely be held accountable in Allah's Court for all of my words and deeds."
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The first sentence of the declaration is about the major beliefs; the second is about deeds; and the third is about a sense of accountability.
All the other things which may be necessary to include in the definition are covered in submitting to Allah's all commands given in the Quran.
