For Pako_Love's C-123
Here https://www.gta5-mods.com/vehicles/fairchild-c-123-provider-usaf-vietnam-war-sp-replace-fivem-z3d

Once installed with their instructions (I had to install it here:  ...\GTAV\mods\update\x64\dlcpacks\patchday12ng\dlc.rpf\x64\levels\gta5\vehicles.rpf), open the dlc.rpf with OpenIV

Follow this path to the file to overwrite:

...\GTAV\mods\update\x64\dlcpacks\patchday12ng\dlc.rpf\x64\levels\gta5\vehicles.rpf\

If you wish to retain the original USAF livery, while in OpenIV, extract the file titan+hi.ytd and titan.ytd. I typically extract the original .ytd to the same folder as the dlc.rpf and rename it to origtitan(+hi).ytd.

When ready to overwrite with one of the C-123 liveries (Con Air or Coast Guard), toggle "Edit mode", top right button, and titan+hi.ytd and titan.ytd from the extracted zip file and drop here:

...\GTAV\mods\update\x64\dlcpacks\patchday12ng\dlc.rpf\x64\levels\gta5\vehicles.rpf\

You might be able to add the new liveries to f104c2.ytd, but when I tried that, it seemed to surpass the size limit for a .ytd file.  You could replace the liveries in the f104c2.ytd file with the enclosed liveries, following the same process, but for f104c_sign_6, for example instead of f104c_sign_1

Just to confirm the livery, I'd click on the titan.yft or titan+hi.yft file to open the OpenIV Model Viewer to see what the model will look like with the new livery.

Once satisfied, close the model viewer and run GTAV and spawn titan with the trainer of your choice.

I've included the GIMP .xcf files in case you'd like to use them as a jumping off point.

Enjoy,
Gillman