How to install Ubuntu on Macbook Pro ?

1. Create a new partition in MacOS / OS X.
2. Install Ubuntu on that partition using installation medium.
3. Use Option/alt key to select the OS during the boot process.

1. Download the latest ".iso" file/image of ubuntu from official ubuntu website.
2. Burn it on spare CD/DVD.
3. OR you can install and use the Etcher (in MacOS) to write it on USB Pendrive and use that pendrive as installation medium.
4. Open the "Disk Utility" program via launcher in whichever MacOS you are using on your Macbook Pro.
5. Create a new partition on storage device (HDD or SSD). I will suggest a size of 20GB+ for new partition. You can use any format, APFS, MacOS Extended. I will suggest "MacOS extended" because it will allow us to create a custom size partition. The format will be overwritten by "ext4" format when we will install the Ubuntu, So, it doesn't matter much regarding what format you choose while creating a new partition here. You can read my article on "How to create a new partition on Macbook Pro?".
6. Shutdown the Mac after all the above steps are completed.

1. Attach the USB Pendrive or CD/DVD containing the Ubuntu image to Macbook and boot the computer by pressing power button.
2. As soon as you hear a sound, press the Option/alt key and keep it pressed.
3. Mac will ask you to choose the boot device after couple of seconds. You can release the Option/alt key here.
4. Select the "UEFI" option in menu and press "Enter".
5. Ubuntu will now load from installation medium and show you some options. You can select the "Try Ubuntu" option to check if Ubuntu works properly on your Mac properly to be on safe side. If you have already tried or made up your mind to go with the install instead, select the "Install ubuntu" option.
6. Installation screen will appear now with some options, such as "Erase disk completely", ..., "Something Else". Since, We created a special partition for Ubuntu installation, We will go with "Something else".
7. Screen will now display the partitions of hard disk. You have to select the partition we created for Ubuntu based on the size and format it by selecting "ext4" as format type. Then, Set it as root "/".
8. You can go forward now and and install the Ubuntu on that partition. it will take around 20-30 minutes for full installation to be completed.
9. After installation is done, Remove the USB Pendrive or CD/DVD and restart the PC.

Most probably, Ubuntu is load by default when you will restart the computer but do not worry, your MacOS is still there.

To switch between MacOS and Ubuntu, You have to use Option/alt key during the boot-up sequence when you start you Macbook Pro. If you keep pressed the Option/alt key after first sound, Macbook will display two options to choose from i.e. Machintosh HD and Efi Boot. Efi Boot contains the Ubuntu and MacOS is in other one. To load the MacOS, you have to select Machintosh HD and press enter key.

The reliability of this approach can assessed by knowing that I used this method myself to install Ubuntu on my Macbook Pro and both OSs, MacOS Mojave, Ubuntu 18.10 are working perfectly.

From my experience, Ubuntu runs smoother and faster than MacOS on mine Macbook Pro mid 2012.