It is quite easy to do this.
Drag one of these folders to the external drive to copy it. (Do NOT try to move a folder between different drives, that could be a recipe for disaster).
Once the copy is done, delete the original folder, and replace it with a symlink.
For example, I will use the Movies folder.
1) In the Terminal, type
"ln -s "
without the quotes, but WITH a trailing space; do NOT press enter.
2) drag the new folder from the external drive onto the terminal window
3) type "~/Movies" without the quotes
4) press enter
The Terminal command could look similar to this:
ln -s /Volumes/MyDrive/Movies ~/Movies