There are many ways of Installing Ubuntu OS, among many, I feel the steps below to be most stable one for both parallel running OS.
Firstly, prepare a bootable USB stick with Ubuntu.
Then, boot the computer from the USB stock and follow the steps below.
Open gParted and create partition as below.
1. First create a boot partition with minimum size 512MB [Set to 1GB in image]
2. Then create a extended partition to the remaining unallocated partition
3. Inside the extended partition
1. Create a new partition for Linux-swap with minimum size 4GB (Just double of your RAM size)
2. Create a new partition for the /home
3. Create a new partition for /
4. Then apply the partitions
5. Set the boot flag for the partition created at step 1 i.e. the boot partitions
1. Finally start the install with a reboot or select install option from the live boot.
2. When asked about what to do with the current OS, click something else
3. This will prompt you to select the drive in which you want to install the new OS
4. You will see that the partitions that we created don't have any mount point
5. Give the respected partition their mount point
1. Click on the partition
3. change partition from “do not use” to “ext4”
4. Check on format
6. Set respected mount point
7. Finally click OK
6. Do the similar to the other partitions
7. Select the boot partition
8. Finally when we are done click on the install