HGSS NEW OVERWORLD SPRITES METHOD

HGSS(in my case SoulSilver)
NEW OVERWORLD SPRITES METHOD - Overlay_1

Take the time to go through every event file in DSPRE to change all overworlds past 993 and before the last one(994-1049) and replace them with their identical Overworld counterparts.
You can determine them being the same if they have the same Sprite ID, located just under the little picture of the overworld beside the properties.
This is necessary so your new sprites don't accidentally appear where other sprites definitely should.
Do this FIRST.
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Duplicate a btx file, from either data/a/0/8/1 or unpacked/OWSprites, that is similar to the type you want to make.
ie a pokemon if you want to add a new pokemon overworld, or a static item like a pokeball for new items, or an npc etc etc
You can use the Sprite ID to find such a file to duplicate, ie 440 is the OW ID for butterfree and 309 is the Sprite ID so the file is 309.
Rename it to the next number after the last one(862, so I named it 863 in my case)and edit with a BTX editor, such as BTX Editor 2.0, to replace the sprite sheet with the new sprite you want to see.

After you finish editing the file to have your new sprite sheet, you'll want to add that new file to data/a/0/8/1 by unpacking it with DSPRE or narchive, adding the new file with a consistent name with the other files in the directory(1_863 in my case), and repacking it with DSPRE or narchive.
Also add it to unpacked/OWSprites with a consistent name (0863 in my case)
Put no file extension on either file, delete it if there is one.
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Replacing the duplicate overworlds 994-1049(56 sprite slots) is simple enough.
Open a decompressed Overlay_1 in HxD or your hex editor of choice.
The table in a decompressed Overlay_1 starts at 0x22F76, so decompress it with DSPRE and DON'T recompress it. EVER.
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Each OW consists of 6 bytes: OW ID, Sprite ID, and overworld type for animation/handling purposes.
Example, OW 994, Offsets 0x22F76-0x22F7B:
First 2 bytes, OW ID, DON'T EDIT THESE. hex+little endian:
E2 03(994)DO NOT EDIT!!!

Second 2 bytes, Sprite ID, edit to new Sprite ID of the file you added to data/a/0/8/1 AND unpacked/OWSprites. hex+little endian:
35 01(309)ORIGINAL ID
5F 03(863)MY NEW SPRITE ID

Last 2 bytes, Overworld type, change accordingly. hex+little endian:
27 4E(pokemon sprite handling, bounces up and down)ORIGINAL TYPE
00 00(no handling, npcs and player work like this)NEW TYPE

Save your changes in Overlay_1 and either reload DSPRE entirely or just switch event files and go back to your map.
Add your new overworld using the OW Entry ID you edited in the table, in my case this is 994.
You should see your new overworld.