How to embroider a snowflake on a knit sweater
This article might be of some help.
A snowflake would be a series of small straight stitches. Sketch out the pattern you want, and either put the sweater section on an embroidery hoop or free hand it, coming up from the wrong side of the material. Don't pull the stitches too tight.
This video shows and explains how to do a duplicate stitch.
The snowflake example below would lend itself well to this technique.