installing a SSD in MacBook Pro mid 2012.

Get an external enclosure supporting USB 3.0. Install the SSD into the enclosure and connect it to the computer. You should be alerted that the external disk is not yet initialized. Do the following:

Drive Partition and Format - El Capitan or Later

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the mfgr.'s ID and size) from the side list. 
3. Click on the Erase tab in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
5. Click on the Apply button and click on the Done button when it is activated.

Upon completion do the following to clone the HDD to the new SSD:

Clone El Capitan and Later using Disk Utility

1. Restart the computer and at or before the chime press and hold down the Command and R keys until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu then press the Continue button.
3. Select the destination volume from the left side list. The volume will be indented below the Container entry.
4. Select Restore from the Edit menu of Disk Utility. A panel will drop down.
5. Select the source volume from the drop down menu in the panel. This is your HDD's "Macintosh HD" entry or whatever you have named it.
6. Double-check you got it right, then click on the Restore button.
7. When the copy is completed you should see a Done button. Click on it.
8. Reboot the computer from the SSD to assure it works.

Destination means the external SSD. Source means the internal startup HDD.

Boot Using OPTION key

1. Restart the computer.
2. Immediately at or before the chime press and hold down the  "OPTION" key.
3. Release the key when the Boot Manager screen appears.
4. Select the disk icon of the SSD.
5. Click on the arrow button under the drive icon.

If all is well, remove the SSD from the enclosure. Remove the HDD from inside the computer and replace it with the SSD. You can put the HDD into the external enclosure so you can transfer your files to the SSD.

I would recommend getting the SSD and enclosure from OWC. They also sell the tools you will need to work on your laptop. If you have an optical drive in your computer that you rarely ever use then get OWC's Data Doubler kit to replace the optical drive with the HDD you replaced. The kit includes the bracket, tools, and instructions.

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.

If yours is a non retina, then I have the same model and installed an SSD after doing exactly what Kappy said. I simply formatted the new SSD while installed in an external enclosure and then cloned my system to it. After testing to make sure it worked, I uninstalled the old drive and installed the new SSD in its place. I also simply put the old drive in the external enclosure to use if needed. You will enjoy the difference in speed! I got my stuff at macsales.com; they also have excellent video instructions. You could also up the RAM while you’ve got it open.

The hard drive cable in the 13" MBPro (mid-2012) is known to have a high failure rate and you may need to replace it.

I personally like using Carbon Copy Cloner when cloning drives as it will also create the hidden Recovery Partition which is necessary for FileVault. The latest version can handle APFS volumes. I've personally encountered issues using Disk Utility to clone a bootable drive and no longer use it.

CCC does not automatically create the Recovery HD partition. You must manually select to install it if CCC asks you to. Disk Utility literally clones both the macOS system as well as the Recovery HD automatically.

I'm adding this comment just for clarification of the difference between Disk Utility and all other utilities such as CCC where it comes to the Recovery HD partition. Time Machine makes an invisible copy of an existing Recovery HD so that you can boot from the Time Machine backup disk.

Kappy in response to HWTech
CCC does not automatically create the Recovery HD partition. You must manually select to install it if CCC asks you to. 

IIRC CCC will automatically create a Recovery Partition if the system is using APFS, but when using HFS+ you are correct that CCC will ask you after the clone has finished whether you want to create the Recovery Partititon. It is also possible to create the Recovery Partition later with CCC if you accidentally click "No".

Kappy in response to HWTech
Disk Utility literally clones both the macOS system as well as the Recovery HD automatically.

Good to know. The last time I attempted to use Disk Utility to clone a boot drive Disk Utility did not create the Recovery Partition which tells you how long it has been since I used it to clone a boot drive. I prefer to use CCC now as it is reliable and I can also deselect items if needed to help speed up the process. Many times I also clone the drive to a .dmg for later restoration and CCC I find is better at this especially if the drive has errors.

My grandson has a device which holds 2 hard drives for cloning one to the other and I'm wondering if I could use this instead of disk utility?

Is that a Mac? If not, don't pan on it working. The cloning software needs to run on a Mac. I think the external is still the best way. That's what I did and I used this kit which came with a decent little USB3 external enclosure:

https://eshop.macsales.com/item/OWC/Y3SSD6E500/

It was very easy using Carbon Copy Cloner. I put the old roto-drive in the enclosure when the job was done and the enclosure is now making automatic bootable clones every morning.

I fully agree with HWtech--replace the hard drive cable at the same time you replace the drive. The cables are available, cheap, and easy to install. However, the 13-inch and 15-inch models use different cables so make sure you order the right one.

You may feel better about doing the work if you look at the install videos in these links:

15-inch: https://eshop.macsales.com/installvideos/macbookpro_15_unibody_mid12/

13-inch: https://eshop.macsales.com/installvideos/macbookpro-13-unibody-mid12/

I found it quiet easy.

toymaker01 in response to babowa
My grandson has a device which holds 2 hard drives for cloning one to the other and I'm wondering if I could use this instead of disk utility? Thanks again.

While this might work, a bit for bit clone is not wise when moving from a hard drive to an SSD as you need to have partitions on 4k boundaries for optimal performance. The bit for bit clone only works when moving from a smaller drive to a larger drive which would also require extending the partition later which adds risk. This is fine if you are trying to retrieve data, but I would never want to do this for regular use.

The best way to clone a Mac drive is using macOS based options. Another option would be to perform a clean install of macOS and migrate your stuff during first boot.

As mentioned, it is quite easy and the video install instructions on the macsales.com site are step by step. They also have the hard drive cable.

And, once you install CarbonCopyCloner, making a clone is simple - just plug in the external hard drive, format it in Disk Utility, and launch CCC.