I've been trying for days now to install Ubuntu 12.04.1 LTS on my mac but it does not give me the option to install it alongside my mac osx. The only options available are to replace mac osx or partition manually, which I have tried and tried to do but I have no clue what I am doing, and there is no easy to follow guide on how to do it. I have at least 20G on my HD that I reserved for Ubuntu, and I don't understand why the option doesn't appear. Help!
To install manually alongside mac OS, first reduce the size of your hard disk partition in mac OS. Then start the installation, choose install manually and create two partitions. One big one type ext4, mount point / and a swap partition the size of your RAM or a bit more (4Gb?).
That should do the trick. The / partition is called the root partition and should contain all your Ubuntu system and your home folder as well.
Since this is your first such operation it is strongly recommended to do a backup of your important data on the drive just in case you mess things up.