So there's basically four steps to creating an Ragnarok Online character set for RPG Maker XP. I won't lie to you. The whole process takes anywhere between one and four hours of non-stop, repetitive work.
The first step is creating your master sheet. A master sheet, a term I've coined, is a Portable Network Graphics (PNG) file with different layers for each pose. Each layer has a white background and measures exactly 256 x 256 pixels. In the simplest of scenarios, you have 8 directions, with 8 walking poses and 1 standing pose per direction. This equals 72 poses total, which means you will have 72 different layers. The rows go in the following order: down-left, down, down-right, left, up-left, right, up-right, and up. Since left and right images mirror each other, you can take advantage of this by copying and flipping layers. The easiest way to get this done is by using a Ragnarok Online character simulator, which will already have the image centered for you. In the end, you should have a perfectly organised master sheet.
The second step is erasing the white space. You won't do this directly on the master sheet. Make a duplicate, in case something goes horribly wrong. Most drawing applications have a handy magic wand tool which will ease your job. Do not merge layers, since we want to handle each pose individually.
The third step, perhaps the most tedious, is accommodating your sprite sheet. To do this, you will need to find out which pose has the largest width, and which pose has the largest height. These values will be your grid values. Your x value will be equal to "width * i + x - (original width - width) / 2", and you y value will be equal to "height * i + y - (original height - height) / 2", where i is pose number, starting from 0. Unless you are Matt Damon in Good Will Hunting, you will most likely need a calculator.
The fourth step, the most gratuitous, is finishing your sprite sheet. I don't like the default Ragnarok Online eyes, so I always change them. Since PNG files take up a lot of space, I advise you saving your image as another file, such as a GIF or BMP file.