First steps to install AspireOS would be to download it and burn the ISO to CD or DVD. Turn on computer and press F12 or similar to select boot device. With netbooks you need external USB optical drive, or you could use Qemu (look bellow).

To use AspireOS in a serious way you must install to internal drive (HDD, SSD, USB) storage device. It is also possible to use AspireOS hosted. You can read more about that at  
Click the install-AROS icon on workbench (desktop) to install. 

With the USB storage device option, you can keep whatever OS that is installed on your computer from before. No need for modifications. 

When AspireOS boots for first time you should configure your preferences. Most important to get right is your input.prefs (keyboard)
Screenmode.prefs (screen resolution) and network.prefs

Don't forget to have a look at the tips and tricks page after installation. Click here

Network configuration

Click on pictures to see how to configure your wireless network.

Don't forget to have a look at tips and tricks page. Click here

Install AspireOS without optical drive.

Qemu guide from Olivier:

It is possible to install AROS v0 i386 on an USB stick from Qemu.
start Qemu with: sudo qemu -cdrom aros-pc-i386.iso -usb -usbdevice host:vendorid:productid -boot d
with vendorid:productid for your insterted USB stick, given by lsusb.
then open tools/hdtoolbox (clicking on it)
from there, prepare a MBR(RDB(DU0)) or RDB(DU0) partition, set as SFS BE. don't forget to set it as bootable. save all. restart the virtual machine (restarting the OS doesn't work)
once on the Wanderer again, install AROS. that's it.

Multiboot with other OS.

First of all you should format disk and leave empty space for AspireOS.
If you want windows to be used together with AspireOS it's quite simple. You run the installer and select "use empty space on hdd" the rest is pretty straight forward. When AspireOS is installed you will get the windows selection in boot menu.

With Linux it's a little more difficult.
You have 2 options. You could install AspireOS with bootloader and edit grub.cfg from AROS.
That option might sound interesting, but the problem is when you update Linux. Your config. will be overwritten. Therefore I will concentrate on option 2.
I will focus on installation with Ubuntu,cother Linux distributions I have not tried.
As said before you must partition HDD to leave empty space. When you install Linux you will be asked where and how you want it installed. You must select manual entry and specify partitions. How much you space you leave for AROS is up to you, but a minimum of 4G is recommended. Set mount point for Linux boot to "/" and use "ext4" file system. Again, leave empty space for AROS, Don't format or do anything with that empty space.
Now boot AROS. How to boot and install AspireOS (look at top of page).
What is important is don't install bootloader from aros-installer. If you do that Linux bootloader will be overwritten.
If you came this far you will have Linux and AROS on drive. The problem now is how to boot AROS.
Edit Grub from Linux is the answer.
You must copy files from from AspireOS live-CD /boot/ to Linux /boot/ copy all the files minus the grub dir from AROS:Boot/ 
From Terminal "sudo nautilus" to get premission to copy to /boot/
If you wish to update AROS installation in the future, remember to copy over the AROS kernel files in Linux /boot/

Open Terminal and type  "sudo gedit" open file etc/grub.d/40_custom
There you will add new entry. Just paste these lines there: 

menuentry "AspireOS (native graphics)" {
multiboot /boot/bootstrap-pc.gz ATA=32bit floppy=disabled enableusb
module /boot/aros-bsp-pc-i386.gz
module /boot/aros-base.pkg.gz
module /boot/aros-fs.pkg.gz
module /boot/poseidon.pkg.gz

menuentry "AspireOS (true colour VESA graphics)" {
multiboot /boot/bootstrap-pc.gz vesa=32bit ATA=32bit floppy=disabled enableusb nomonitors
module /boot/aros-bsp-pc-i386.gz
module /boot/aros-base.pkg.gz
module /boot/aros-fs.pkg.gz
module /boot/poseidon.pkg.gz
Open terminal again. Type "sudo update-grub"
Reboot and hold shift. You will now see bootmenu with the AspireOS entry.
Select it and AspireOS will boot.