Can't. I'm at work at the moment, but I guess I can try.
And oh, make sure you're using A-Map v1.92. Don't use the beta 1.95 version.
It'll completely destroy your ROM if you're not careful.
Anyway...
1. Open your ROM with A-Map.
2. Click the [Block Editor], or that purple jigsaw puzzle piece.
3. Select the correct palletes from the Pallete drop-down menu.
4. Click the [Picture] tab and select [Save Tileset 1].
5. Open your saved tileset with MS Paint.
6. Paste your new grass tile into the tileset.
7. Color your grass tile with the same available colors the tileset has.
8. Save.
9. Back to A-Map, go to the [Block Editor] again.
10. Click the [Picture] tab and select [Load Tileset 1].
11. Done.
Oh, and you might need to uncheck the [Automatically look for offsets] option in the [Settings] tab of A-map. That way, you can manually provide the offset you want to write with.