It's a matter of what tools you have to work with. RMXP is likely the one you've got, so just use the advice given by Quilava. That said, try to make the background of your sprite sheets a color that you don't use. (In my opinion, if you're ever spriting, a good best practice is to never use flat white or black, instead colors #f0f0f0 and #0f0f0f, so that you know it's not there by mistake.)