Make sure you have the right sized image - so if your map is 20x10 tiles, it'll be a 640x320 image.
Then you want to set up the guidelines/grid choices in the program to show the correct grid that would be used in RM (if ace, 32x32). Then place tiles according to that. ^.^)b
It might help to make a quick map with general details of what you want where, then take a screenshot of it and use it as a background in the image program, then just keep making layers for different things over the top of it - say, one layer for each house, one for the road, one for decoration, etc. Then remove the layer that was the quick map so that it's not shown up.
After that split up the images on separate layers depending on their 'height' (images that are supposed to be above hero on one layer, shadows on another, same-as hero images on another layer, below hero on yet another... and so on).
Then copy those layers to different images and save them, then use them as images. There's bound to be better tutorials out there. I'm assuming you know a few of the basics about images in RM and the like.