Tā sīxià yí yè biànjiān, xiěxià le zìjǐ de diànhuà hàomǎ.
She tore off a sheet of notepaper and wrote down her phone number.
Tips
usage
便笺 is a written word for a short, informal note or the paper it is written on. A self-stick note is a 便利贴. Here 便 means 'informal, handy', the reading in 方便, not pián as in 便宜.
