Allotments 4 All

My first earlies are showing signs of life with 2 inch shoots above ground. I know I'm supposed to cover with earth, but do I do this now or should they be taller. How much leaf are they supposed to show and how much am I supposed to cover... and when.  Dont know what the balance is between cover and uncover.
Thanks to all for the responses.

Cover now just in case you get a frost.    ;D ;D ;D