Connect know representations of numbers, such as two counters, two fingers, or the utterance two, to written numerals and number names. Break this skill into multiple sessions. I really like the way JoAnn does it, as seen here. Write by tracing at first, then without tracing.
Conclude by giving your students this challenge:
Buzzy Bee by NRICH
K.CC.A.3: Write numbers from 0 to 20. Represent a number of objects with a written numeral 0-20 (with 0 representing a count of no objects).