to note down /noʊt daʊn/ /nəʊt daʊn/
to write down /raɪt daʊn/ /raɪt daʊn/
to jot down /ʤɑt daʊn/ /ʤɒt daʊn/
make a note /meɪk ə noʊt/ /meɪk ə nəʊt/