Hi
@Kevin Boyle so youre RudeDawg over on the discord, welcome!
The concept for all skills is the same - all files a skill needs to work correctly must be made available to it. This is easy for vanilla skills (as you saw) because its files are "always there". Since the same isn't true for custom skills, you'll just need to copy over all the stuff that it's looking for.
To find out what a skill is looking for a little detective work is needed...
1. The main skill .DAT has a lot of clues. The top part are main settings which are standalone but there are a few things which are external to the .DAT like...
1a. The ICON it wants to use - If just a vanilla icon there's nothing more you need to do. But if it's a custom icon you'll need to find and copy over the icon image file AND its .IMAGESET file too. Those are usually in MEDIA/UI/ICONS/SKILLS
1b. The ANIMATION it wants to use - If just a vanilla animation there's nothing more you need to do. if it's a custom animation and you want to use it, you'll need to find and copy over the animation .SKELETON and edit your class' .ANIMATION file to include that particular animation. Look for the .SKELETON in MEDIA/MODELS/PCS/(folder of class)
Note - i suggest you just forego any custom animations for now and specify a vanilla animation in the skill .DAT - that will make your life much much simpler

2. Still inside the skill's .DAT are the [LEVEL] blocks for each level of the skill. These blocks specify a lot of external resources the skill needs, such as...
2a. .LAYOUT files - these files do a range of things from specifying the area within which the skill works to firing off particle systems to make our skills prettier! 99% of the time, .LAYOUTS are called by their complete path-and-filename, so they are very easy to find and copy over
2b. EFFECTS - demarcated by [EFFECT] blocks, the <STRING>TYPE: within that block is the most helpful clue. Usually skills will use straighforward effects like DAMAGE BONUS in which case nothing to copy over. But if it's a SET STAT effect that's more complex - i'd encourage you to read up on that
here tho further discussion will still be needed / lets save that for later k?
2c. AFFIXES - demarcated by [AFFIXES] blocks, the <STRING>NAME: within that block is what's important. That's the AFFIX name which you will have to search for in the source mod's MEDIA/AFFIXES folders and copy over. Note that it's also possible the skill, tho custom, is using a vanilla affix in which case there's nothing to copy over.
Okay thats like 80% of what u need to remember - give it a shot and we can chat more later - i gotta go get to some chores