@InZanity Submissions are always open at submissions@tapasmedia.co, so you can submit anything at any time for review, so a completed manuscript is fine to send, but it would not qualify for the 1st place prize specific to the challenge.
@Mane A story you've started but have yet to submit is the perfect project for this challenge as long as you do put work into it during the month of January. If you are submitting specifically for the challenge on January 31st, please do have it separated into episodes (not necessarily 400 words each, but divided at natural cliff-hangers, chapter endings.) If you want to submit earlier or later than January 1st, you won't qualify for the 1st place prize, but will go through the normal submission process with potential to publish.
Hope this helps!