MSPaint is fine. Others would use IDraw. Either way..
Each file is 144x384 pixels, and each "frame" is 48x48. (Obviously, this means 8 rows, 3 collumns.) From there, you would just draw whatever you want, with 3 frames of animation for each pose. Open up a current RTP one and use Save As to make a copy somewhere so you can mess around with it.
Make sure it's 256 colors! (If you're having trouble importing it, just Save As a 256 color bitmap in MSPaint.)
Don't forget to set up your battle charset when you're done (Animations 2 tab in the Database). If you don't know how to set those up, look here: http://zelaron.com/forum/showthread.php?t=27929
Each "pose" is made up of 3 frames. You'll need a pose for each of the required ones - check the Animations 2 tab, Poses 1-12 since I don't feel like typing them all out here. You can reuse animations if you want.. by default, in fact, Skill Use and Item are both identical. Just fill up the required poses with whatever character you're making.
Check out the way RPG Maker's RTP does it - 2 Battle Charsets, so there's more than 8 animations for it to use. They also give you a good feel for the layout of Battle Charsets in general.
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Also, don't double post - if you need to say something right after you've already said something else - edit your reply. That's just one of Zelaron's standard rules.