You need to install from the CD or usb and it will be the same as a normal
installation except:

1) choose manual or custom (the last option, they have changed the wording)
when it comes time to choose where to install. The first option will be to
use the entire disk, etc/

2) it will show the drives and partitions in graphical format, listing sizes
and format, NTFS, ext4 etc. It will name the partitions using the Linux
method which is sda1, sda2, sdb1, sdb2, etc.

If you need to partition you have the option of doing it first or doing it
here. I suggest that you do it ahead of time to reduce the stress by
spreading it out.

You don't say what your setup is, one drive or two or whether you have
anything other than Linux. We need that info to tell you more precisely what
to do. If you have not partitioned then stop here and let us know more. See
below *

3) you need to choose to use a partition for root, one for home and one for
swap. Swap will be tiny, no more than equal to your RAM if you have lots and
no more than double your RAM if you are short of RAM. Remember RAM is faster
than disk access so too much will slow you down as much as too little. Root
is typically smaller than home. Root can be anywhere from 8 GBs to 30 GBs
depending on your habits. You won't max out by installing applications if
you go toward the lower end, but if you can find ripping DVDs and making
videos are restricted by space problems. You do not want to run out of space
on root. Been there, done that. It isn't pretty. Home will be the largest
that you can make it in the rest of the space. Use it all.

4) You need to tell Ubuntu three things about each. You need to say which to
use (default is not use). You need to choose a mount point. Use / for root
and /home for home. Swap does not have one. You need to choose a file
format. Use ext4 unless you have reason to do otherwise. You format all 3
partitions the FIRST time. On subsequent installations you will NOT format
home. The rest will be the same. You edit by right clicking on the graphical
ribbon of the partition you are using (or left clicking and choosing Edit
button below) and it will bring up a box with those choices.

Then you commit the changes, paying close attention when it presents the
screen to review the changes. After that it is too late to change your mind.
The rest will be a normal installation.

* A word on partitioning. Partitioning is quite straight forward if you
proceed in steps. First you need to Resize by shrinking the drive to make
space for a new one. The new space will say unallocated. You need to plan
whether you put it at the end or beginning. This decision depends if you
have anything on the drive and you want to preserve the order. If you have
Windows on the drive then make sure it is first. Then you create new
partitions in the unallocated space. You just right click and choose new. Do
this according to your plan, leaving enough unallocated for the remaining
partitions. You are limited to the number of primary partitions, so you can
make and extended partition and more primary partitions inside one. I am
hoping that your setup is simple so we don't have to get into this. You will
need three Linux partitions counting swap plus whatever else you will want
for Windows or data. Keeping the total number of partitions to four or less
is easiest.

Read this first: https://help.ubuntu.com/community/HowtoPartition