How do I create the [more inside] jump when posting?
The break and the "more inside" text are created for you automatically after you fill in the posting page form. There are several boxes to fill in when posting. On Mefi, the title and the box called "Description" will form the part above the fold (before the "more inside") which appears on the main page; the "Extended description" forms the part below the fold (after the "more inside") which appears after a person has clicked on your post.
On AskMe, the question title and the box called "your question" will be the part that appears on the main page; keep this part brief. The next box called "extended explanation" will be the [more inside] part of your question and can be longer. Again, the words "more inside" will automatically be added to the end of your question.