Well, as a general overview; you'd want to use Content Patcher to do this:
https://www.nexusmods.com/stardewvalley/mods/1915
The Content Patcher documentation: https://github.com/Pathoschild/StardewMods/blob/stable/ContentPatcher/docs/author-guide.md#readme
Extra info: https://stardewvalleywiki.com/Modding:Content_Patcher
I'd start with the intent of your mod.
If those are the only 2 items you want to retexture. You can simply create your own sprites, & add them into a 16x32px PNG image (16 pixels tall, 32 wide[for 2 items]).
However, if you plan to do a lot more item retextures down the road. I'd recommend matching the size of the item tilesheet. As it makes things easier down the line.
Location: Content/Maps/springobjects.*
Current Size: 384x624 px
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From there it's just a simple edit image action. Where you match the 16x16 sprite you drew; to the location you want to overwrite.
Here's a partial code example from my Tea mod:
Code:
{
"Action": "EditImage",
"Target": "TileSheets/crops",
"FromFile": "assets/MysticTempest_Tea_Sprites.png",
"FromArea": { "X": 0, "Y": 0, "Width": 128, "Height": 32 },
"ToArea": { "X": 0, "Y": 1568, "Width": 128, "Height": 32 },
},
Target:
This is the file that you want to edit.
In your case it'd be "Maps/springobjects"; which contains all the item objects
FromFile:
This is the name of your own spritesheet or datafile.
I put my art under a folder named "assets".
FromArea:
This is where you open up your preferred image editor, and find the location of your art.
The X, Y value of (0,0) = Top Left corner of the image file.
So, in this case I'm telling it to start at the top left corner.
Width & Height is how much of an area you want to copy from your image.
Now, Content Patcher is pretty flexible. You can do all sorts of custom sizes. In my case, I copied a whole row of crops. Each individual crop tile is 16x32, but I did 128x32 for the whole row.
Since, you're only editing the 2 pendants. You should do them one at a time. At 16x16px each.
ToArea:
This obviously is where on the 'target' you're editing.
In my case I added to the tilesheet as I was adding new items.
For you; you'd want to find the locations of the mermaid pendant & void pendant. That will be your ToArea target.
-------------
A little hint to get you started:
Pic example of the "Maps/springobjects.png" file:
If you see where my cursor is; the black pixel. And, then look at the bottom left; it says "(0,304)".
That is where "Algae Soup" starts.
Now you can continue counting over in 16pixel increments to find the starting location of the mermaid pendant.
Once you find their respective locations. You can create 2 'EditImage" actions, and match your sprites to the game's tilesheet.
You'll need to fill out the rest of the info for the ContentPack, but the links above has some more info on that.
Hope that helps a bit.
