Welcome on forum!
When I started so I took the DCS World OpenBeta \ CoreMods \ aircraft \ FA-18C \ Liveries \ FA-18C_hornet \ Any skin
I put that in folder with my texture and just changed names
For example, from "F18C_DIFF_UK" to "F18C_DIFF_your texture name"
And copy your skin to f-18 liveries and its done.
I do not know how to explain it better