First of all, make sure to have an up to date backup.
For the purpose of this example, let's assume you are doing this with the Music folder, and that you are using an external drive called "MyDrive". To make things more organized, we are putting the Music folder inside a "MyStuff" folder in "MyDrive".
1) Create the "MyStuff" folder inside "MyDrive"
2) Drag the Music folder to "MyStuff" to copy it. Depending on the size of the folder and the speed of the drive that may take a little while (but since we are talking about SSD it should not be very long).
3) When the copy is finished, delete the original Music folder [you did make a backup, didn't you?]
4) Open Terminal.
4a) Type "ln -s " (without the quotes but with the trailing space!). Do NOT press enter
4b) Drag the newly created Music folder from the external drive onto Terminal. Do NOT press enter
4c) type "~/Music" (again, without the quotes)
4d) Press enter
This last step creates the symlink. Now for all intents and purposes, it appears that you still have a Music folder in your home folder, but it is quite transparently pointing to the real location.