You write the entire post inserting the <!--more--> tag in the spot you want the front page entry to end. If you use the visual rich text editor then you use icon #13. If you use the standard editor you click the [more] button and WordPress technology takes care of the rest. There's nothing else you need to do to put the rest on another page. The software will insert the tag at the end of the excerpt and put the entire post on another page for you.
This tag is meant to be used for long posts and not for every post. So be aware that some readers, like myself, are not impressed by those who try to create a second click stat for every post they publish in this format. I do not click through. And I do not return to read blogs where the blogger employs use of the more tag habitually to strategically secure a second click stat.