Q:

How to set up a separate drive for documents/users?

I am going to make a dual boot install of Ubuntu. I have two drives, 1 ssd and 1 hard drive. I will install /root, /home and swap on the ssd as people say that will be faster. Then for documents they recommend using the hard drive, but I cannot find information of how to do that configuration.
How do I set up the hard drive for documents? How will the system know that the default location of folders like Documents and Pictures is on a separate drive?  Does the user folder goes in the hdd or is the ssd, how do I set that up?

A:

Some basic information:
The default configuration is that all system folders are installed on a drive together under root ('/'), namely,
/
|-- bin
|-- boot
|-- cdrom
|-- dev
|-- etc
|-- home
|-- lib
|-- media
|-- mnt
|-- opt
|-- proc
|-- root
|-- run
|-- sbin
|-- srv
|-- sys
|-- tmp
|-- usr
|-- var

You can check more details about the file system on Ubuntu Community Wiki here (Linux FileSystemHierarchy).
Now each of those folders can be residing on different physical drives which is assigned while installation. It is during the installation that one can specify where to place the /home and /tmp etc... under the partioning option.
Now, /home is important since inside this folder are the folders for each user, like 
home
|--/user_1
|--/user_2

And then under each user,
user_1
|--/Desktop
|--/Documents
|--/Pictures
|--/Downloads
|--/Videos
|--/PublicHTML
|--/Templates
etc...

So, it comes down to the installation part and there you can ONLY define the drives where the folders under root ('/') can reside, hence you can define where is /home, and then the folder for the user and the following directories for documents and other go under it (on the specified drive). When installing you can choose the Specify manual partioning (advanced) and there you have to one by one specify the folders (home, var, tmp, etc, lib, etc..) and their location and intended size. Be careful while specifying size. You can check this link for some images Manual partitioning-HowToGeek and steps.
The most straightforward way is to keep your /home in the HDD while other folders stay on SSD. Since recently accessed files are cached (and caching is very efficient on Ubuntu and linux) you will not see much decline in speeds with HDD while still having a fast OS on SSD.

Case where installation is done and you need to move :
If you want to move, for example, home folder to another drive after your deafult installation on one drive, it is possible. Follow this link  to understand that, Moving home folder - Ubuntu Community Wiki and on askubuntu - moving home folder.

If you deliberately want to keep a folder on SSD (a second SSD).
You will however need a second SSD. A new SSD (or any other drive) can be mounted to the file system and by default the mount point is under /mnt. You can change the mount point to suit your needs and then use this folder for your documents or intended files.
You can try by mounting the drive (SSD) inside a folder located under your /home/username , for example, under /home/username/fast_folder using fstab. Read about fstab here fstab-community wiki.  Next, you can set the default storage location for applications like  LibreOffice, ImageMagick or Gimp to this fast_folder. This way you have a storage located on an SSD.

