Citrix XenApp 6 requires Windows 2008 R2 servers. If you have only Windows 2008 or older servers then Citrix XenApp 5 is the latest version. The installation software can be installed from Citrix.com and you will need the customer's Citrix login information and what they are licensed to use.
There are four main components that we typically require for a basic Citrix installation, Application server, license server, Secure Gateway and Citrix Interface web server. In theory, we can have all of these running on the same server but our preference is to install at least the web server and Secure Gateway on a separate server.

Pre-installation recommendations:

Put the Citrix application server in Remote desktop Services mode prior to doing the Citrix Installation. There are other Wiki articles on doing that.
XenApp 6 does not support a MS Access datastore. It does support, MS SQL Server and Oracle.
For an Oracle datastore connection you will need to install the 32-bit Oracle Data Access Components (ODAC) on the Citrix application server. THIS HAS TO BE INSTALLED PRIOR TO DOING THE CITRIX INSTALLATION. The ODAC files for Oracle 11 can be found at http://www.oracle.com/technetwork/database/windows/downloads/utilsoft-087491.html.

Minimum Requirements posted by Citrix for a 10g\11 installation on an Oracle DB server.

The server hosting the Oracle database should meet the following minimum requirements.
• There should be approximately 100MB of disk space for every 250 servers and 50 published applications in the farm. The required disk space increases if a large number of published applications are in the farm.
• The Oracle Client must be installed on the Citrix server before you install Citrix..
• Oracle supports both Windows and Oracle authentication. Use Oracle authentication.
• The Oracle user account must be the same for every server in the farm because all servers running Citrix Server share a common schema.
• The account used to connect to the data store database must have the following Oracle permissions:
• Connect
• Resource
• Unlimited Tablespace (optional)

Installation Phase

Download from Citrix.com the appropriate installation file in a compressed format. Un-compress the downloaded file to a unique folder on the Citrix server and run the autorun.exe file. This will open an installation menu, from which you will select "Install XenApp Server". With XenApp 6 it will prompt to install any missing items that are needed for the installation. Older versions of XenApp or MetaFrame will stop the installation if the other required software is not installed. If that is the case you will have to stop the installation load the missing software and restart the installation of Citrix.

Citrix XenApp 6 installation menu:

After selecting "Install XenApp Server", If ".NET Framework 3.5 SP1" is not installed on the server you will receive a pop-up asking to install the software. Click OK to start the installation.
If the installation of ".NET Framework 3.5 SP1" is successful, you will be returned to the installation menu where you will once again need to select "Install XenApp Server".
This will start the Add Server Roles installation and you will be prompted for the "Edition" of Citrix you want to install. The client should be able to furnish what edition they are licensed for.
After you select the edition and accept the license agreement you will be prompted to choose the "Role" you need.
If you are separating out the various roles to separate servers then check just the role you want to install. In the diagram above it shows that we will install the License Server, XenApp and Web Interface all on one server. We recommend at a minimum to place the Web Interface and Secure Gateway on separate servers. Hopefully discussions with the client will determine what software\role is installed where.
With a XenApp 6 installation it will install any missing needed software prerequisites needed for the particular role. Just follow the prompts for the installations. Reboots maybe required depending on the components that need to be installed or adjusted. If you need to reboot the XenApp server Manager should be displayed. You will select "Resume Install".
Hopefully after all the reboots and installation restarts the software installs correctly. XenApp 6 does perform better on installations than previous versions.
After the installation you will have to configure the various roles. As our example server we have installed XenApp, Web Interface and License server all on the same server. If you have separate servers for each role the installation and configuration is the same but limited to just the role selected.
We will start with the license server. Unless there is a very specific need do not change the port assignments for the license server. If we need to troubleshoot an installation have a common install helps.
The license server default account is Admin and this is only for access into the license server software. If you install the license server on a separate server then make sure the ports are open between the license server and the Citrix application server.
We want to create a new server farm and we want the first administrator account to be either a local server administrator or a domain administrator if the server is in a domain. We DO NOT support Citrix in the CUST domain. The client will have to have their own separate domain. The domain conversation should have been reviewed with the client prior to beginning this installation.
After this screen it will ask you to supply the Citrix license server information. If it is not installed yet you can add this information later.
XenApp server configuration is typically the most troublesome, as this is where the database connections are first used. If you failed to install the Oracle ODAC components IT IS TO LATE NOW! You can probably get a SQLPLUS connection to the Oracle database but Citrix will not see the link properly.
If we are using an Oracle database and we properly installed the ODAC components there will be an option to select an existing Oracle database as well as the SQL databases the screenshot shows. You will need to select the Oracle option if it is an Oracle database.
The database credentials shuld be the information supplied by the dba when they created the database be it either Oracle or SQL.
The next screen is to configure Shadowing. Typicaly we allow shadowing but we force an acceptance pop-up window and we log the sessions. This should be discussed with the client as it is hard to modify after the installation.
For a Citrix Zone you can give it a uniques name or just accept the default.
Review the summary screen and if all looks correct then apply the settings or else use the back button to make any needed changes. Once the XenApp server is configured there is a Finish button to select. Once selected, reboot the server.
If you have installed selected the web interface role for this server as well, then after the reboot you will need to access the Citrix XenApp Server Role Manager again to finish the web interface installation.
Select "Configure" under the Web Interface Role tasks.
Select "XenApp Web" in the Web Interface console to begin the configurations of the interface.
Set the path, accept defaults if youwant
Authentication will take place at the web interface.
Confirm the settings and select next. You should see a successful site creation message.
Now to configure the site
Set the farm name as the same name you used to create the farm. Leave the other settings at default for now.
Settings to select for the next few windows:
Authentication will be "Explicit"
Allow any domains
Login screen appearance will be "Full"
Resources available will be "Online"
On the summary screen select "Finish" and you should get a screen similar to this:
Role Manager should now show all configured.