Pick a name for your new entry, substitute that in the link at the end of this line, and preview the following subst on its empty page:
{{subst:sjk-today|TOPIC}}
Then add your post to the top of the subst'ed text. Day and month pages are auto-linked from the timestamp; if you need to create them as well, they'll be red. Use {{sjk-day|NAME OF DAY}} at the top of a new day page.