Copy the folder, keep the name
Place the directory so the folder name still matches the name field inside SKILL.md. Choose project scope if the skill encodes rules about one codebase, or user scope if it encodes how you personally work. A skill installed in the wrong scope is not broken, it is just present in sessions where nobody wanted it.
Read the skill before you install it
A skill is instructions an agent will follow, and it may carry scripts it is allowed to run. Installing one you have not read is closer to running a shell script from a stranger than to installing a theme. Check what it tells the agent to do, check what is under scripts/, and prefer skills whose source you can see.
Confirm it loaded, not just that it copied
The failure people report as install trouble is almost always a name that does not match the directory, or a description too vague to match any task. Ask the agent to do the exact thing the skill is for and watch whether its behaviour changes. If nothing changes, the problem is the description, not the file location.