Organize/Use 2 hard drives

Erase the SSD and Install macOS using Network Recovery:

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible back up your files before proceeding.

1. Restart the computer. Immediately after the chime hold down the (Command-Option-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Install OS X and click on the Continue button.

How to use an SSD with your HDD

If you are going to use an SSD as a boot drive together with your existing HDD as the "data" drive, here's what you can do. The following assumes you have an installed and working system on the HDD.

After installing the SSD you will need to partition and format the SSD using Disk Utility. Then, install OS X on the SSD. After OS X has been installed boot from the SSD. Use Startup Disk preferences to set the SSD as the startup volume.

Open Users & Groups preferences. Click on the lock icon and authenticate. CTRL- or RIGHT-click on your user account listing in the sidebar and select Advanced Options from the context menu. You will see a field labeled "Home dir:" At the right end you will see a Change button. Click on it. In the file dialog locate the Home folder now located on the HDD (HDD/Users/account_name/.) Select the folder, click on Open button. Restart the computer as directed. When the computer boots up it will now be using the Home folder located on the HDD. 

Another more technical method involving the Terminal and aliases is discussed in depth here: Using OS X with an SSD plus HDD setup - Matt Gemmell. This is my preferred approach because I can select which of the Home's folders I want on the HDD and which I don't want. For example, I like to keep the Documents and Library folders on the SSD because I access their content frequently.

Be sure you retain the fully bootable system on your HDD in case you ever need it.

Kappy has provided great advice, as usual.

The only thing I would like to add is that in general, when moving the Home folder to a secondary drive (not the boot drive) you should have at least one Admin User, whose purpose is only occasional Administration, and leave it firmly on the boot SSD (not moved).

If you don't already have such a user, create one and give it a password that will be obvious to you six months from now (preferably not a dictionary-word), and/or create a password hint that will help you remember.

The reason for this is to make certain that if the drive with your regular home folder fails to Mount for any reason, you can still log in without that drive present to debug the situation.