What is best way to teach a 6 year old how to write?

Practice is the only thing.

Practice the necessary movements (circles and so on) first. Drawing pictures and playing in fingerpaint will help.

When the child is ready to practice actual writing, add extra methods to pencil and paper practice. Sticks in wet sand, fingers in a tray of cornmeal, a drinking straw in soft ice cream; whatever you can come up with that tickles your fancy. The more you interest the child, the more s/he will want to practice; the more practice s/he has, the easier it will be to learn.

Good luck!

Make sure to keep any learning experience fun. That goes for any age. If it is a chore with much correction, they will turn off to the learning process. Validate the efforts made, even though not perfect for their age. 

Be sure to help the child understand the purpose of writing. Lots of love and help goes a long way and is as important as technical proficiency.

I'm a homeschooling mom, and to teach my daughter how to write I used Zaner-Bloser's handwriting instruction. But, there's a lot more to teaching handwriting than just practice. Another answerer added the idea of drawing and using fingerpaints or sticks in the sand.. which are all great ideas! There's also the added value of technology being incorporated into handwriting instruction. Zaner-Bloser has a multi-media approach which is appealing to young learners.