Found this one confusing every time I needed it, so decided to write it down once and for all.
You want to link a copy of a projectfolder with a website in your htdocs folder of MAMP. This way, you can keep all your projects together in one folder and still serve that project in your htdocs folder.
Your project folder (e.g. /Users/username/Documents/work/projectX).
A folder to put the symlink in (e.g. your htdocs folder in MAMP: /Applications/MAMP/htdocs).
ln -s /Users/username/Documents/work/projectX /Applications/MAMP/htdocs
First directory is the source folder, second directory is the target folder
No trailing slashes behind the directories
(/Users/.../projectX, not /Users/.../projectX/)
Use absolute paths, not relative to your current folder location