First of all, since a manga page is supposed to be printed and trimmed afterwards, we have to be careful of our layout.
For printing, a page is separated into three areas:
The bleed area will be cut off 95% of the time, so make sure not to have anything important there.
The trim area is where the page will be trimmed (who would have guessed XD), so there‘s a chance that something of it will be cut off. Therefore it should not contain any truly relevant elements like text or characters faces, etc. either.
The live area is the main part of the page. Everything important goes here.
Keep in mind that all publishers and print-shops have their own specifications, better check them out for the exact format before you start drawing your pages.
