I tried to realize exactly that on my raspberry pi some time ago..
I dont remember exactly how I did that and cant find the "source" of my information anymore but it should roughly go in this direction:
Create a user without giving him an own home directory (ftp_user is his name, you can give him a different one):
sudo adduser ftp_user --no-create-home
(Here you should be able to give that user a password)
If you want ftp_user to be able to access a specific folder you need to
mkdir /path/to/specific/folder # create a folder
sudo usermod -d /path/to/specific/folder ftp_user # assign to ftp_user
Then you need to give that user the ownership of that folder
sudo chown ftp_user:ftp_user -R /path/to/specific/folder
And In general the rights for that special folder should be like this:
sudo chmod 774 -R /path/to/specific/folder # You:rwx Group:rwx eveyone else: r--
sudo chmod 775 -R /path/to/specific/folder # You:rwx Group:rwx eveyone else: r-x
In case the folder is in /var/www/.. it may be neccessary to add the user to group www-data or something like that (if I remember correctly there is a www-data group also)
What is the www-data user?
and of course chown that file respectively:
sudo usermod -G www-data ftp_user
sudo chown ftp_user:www-data -R /path/to/specific/folder
Same for your home directory, if you want to share your home folder with that ftp_user you should be able to add ftp_user in your group but then you shouldn't chown your home folder but disallow write executions for group and everyone else..
Be very cautious with the ownership of folders and permissions.