I finally made my first new mesh. Thanks to
Qingshuangtongzi's tutorial, Yilixiya's tutorial and Zhongyuan's guidance! (Qingshuangtongzi is a great creater and only shares his works at gamesir.com; Zhongyuan is also a great creater, you can find her creations for both Sims2 & Sims3 at XM sims or MTsims.)
Your sims can wear this bowknot together with many clothes, but it is not suitable for some clothes when your sims get too fat.