Open a .FMP in Mappy (V1.3.18 works well)choose an already made .fmp or one you made from the provided example small.FMP
Edit the stage as you wish with the tileset. Maps size from 20x15 - 100x100. It is with and height.
(70 with - 50 height example for battlefield stage)
To edit Map size, click on edit, then click on rezise maps. (or press ctrl+r)
Once stage is compete, click file, and then click export. Leave settings as is and only click and checkmark Data as text (for GBA). Then on the next screen uncheckmark everything except for Anim and Block, Map Array, 2D format and this layer only. Click OK when done. Now look for the newly created txt file under the same name as your stage .FMP name. Click on the text file. Look in the file, it should have something like this.

/* V1.3.18, please see mapwin.htm docs 'Exporting map for GBA'
 * for header defs and functions etc
 */

#include "gbamappy.h"


BLKSTR blockstr[90] = {
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0x00 },

Notice the BLKSTR line followed by alot of numbers.
Scroll down the text file until you see this,

short maparray[51][71] = {
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 

This is your stage map your are looking for. Erase all the BLKSTR text so your text file should match exactly this in the text file.

/* V1.3.18, please see mapwin.htm docs 'Exporting map for GBA'
 * for header defs and functions etc
 */

#include "gbamappy.h"


short maparray[51][71] = {
{ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 

Short maparray should be on the same line as BLKSTR was. Not sure if needed, but looking at the example txt file from stage small01.arn youll see that after maparray there is a zero number next it.

short maparray0[39][59]

it works in ppsspp with and without the 0 next to maparray, so its only if you want to make sure your stage goes exactly like the example stage text file if you want to do so. Thats it! Now you can make new stages for Smash GP!