Nice Artcile by
Michael Mukalian explained step by step
I just got done doing this a couple times, so I wanted to just throw out
there steps with pictures. The installation process is pretty much
from the
Deploy Office Web Apps (Installed on SharePoint 2010 Products) TechNet article, but I wanted to show images along with each step. So, with that said, here we go:
1. Download the Office Web Apps install from wherever you've access to
it (it's a seperate download in the MSDN area), and then run
setup.exe.
2. Enter your Product Key when asked for it
3. Accept the Software License Terms and click <Continue>
4. Select a file location then click <Install Now>
5. Watch the progress bar scroll :)
6. When the installation completes you'll be prompted to run the
Configuration Wizard. Make sure the checkbox is checked to do so, and
click <Close>
7. Click <Next> on the Welcome dialog
8. Click <Yes> in the Service Reset Notification dialog
9. Click <Next> in the Completing dialog
10. Watch another progress bar :)
11. Click <Finish> in the Successful Configuration dialog
12. Alrighty then, the bits are installed so time for configuration...fire up Central Administration from
Start -->
All Programs -->
Microsoft SharePoint 2010 Products -->
SharePoint 2010 Central Administration
13. Click the Manage services on server link in the System Settings section>
14. You'll note in the Services page that there are now instances of the
services that represent the ones necessary for Office Web
Applications. These will need to be started up. Click Start on each one to start them up.
- Note: When installing Office Web Apps in a
multi-server environment, these will need to be started on each server
that you choose to run them on.>
15. Now that the instances are started up, we'll need to create the
service spplications, and their proxies, thereby wiring up the Web
front-ends to the service applications. So from the home page of
Central Administration click the Manage service applications link in the Application Management section>
16. In the Service Applications page, when you click the <New>
button, you'll see a list of Service Applications types to create.
Note that you now see the ones relevant to Office Web Apps. These are
the ones we now have to create. So, from this dropdown, click the Word Viewing Service option. The Word Viewing Service Application dialog appears.
17. In the Word Viewing Service Application dialog you should enter:
- The Name of the service application
- Determine if you wish to use an existing application pool, or
create a new one. In this instance, use the existing Default
application pool
- Make sure the checkbox is checked to add this service application's proxy to the farm's default proxy list
- Click <OK> when complete
18. Now that you've created the service application for the Word Viewing Service, you'll need to create one for the PowerPoint Service Application.
19. Now, depending on how you originally installed SharePoint 2010, you
may or may not have to create a service application for the
Excel Services Application.
If you installed via the wizard-based GUI, this will already have been
created. If you used a scripted install (say, via PowerShell), you may
not have created one for Excel. So, if one doesn't yet exist for Excel
Services, create one.
20. Once all the service applications have been created, you'll need to
activate the Office Web App features on the site collection you want to
access them from. Note that this is a site collection-scoped feature,
so, go to your site collection's top-level site, and from its
Site Settings page, click the
Site collection features link in the
Site Collection Administration section
21. Here, you want to activate the
Office Web Apps feature first
22. And then activate the
Open Documents in Client Applications by Default feature

23. Now, when you mouse-over an Office Word document, PowerPoint deck,
or Excel Spreadsheet you'll see the following options in the dropdown
menu
Now, you're able to view and edit Office content within the browser.
Click the images below for examples of the in-browser Excel, PowerPoint,
and Word features.
Excel
PowerPoint
Word
Additional Items to Consider
While you can go through the UI to perform the activation of the
features on a site collection, when you have multiple site collections
going through the UI is not only tedious, but a bit time consuming. You
can create a PowerShell script and use the Enable-SPFeature to activate
features via script. So you could loop through all the site
collections in a web application and enable them that way.
Also, when creating the necessary service applications, you can also
create specific application pools (along with designated managed
accounts) to run these service applications. Keep in mind that whatever
new accounts you use (if not using the default) that they need to have
read/write access to the SPConfig and all content databases that house
the site collections where Office Web Apps was activated.
Reference:
http://www.mukalian.com/blog/post/2010/12/11/Installing-Office-Web-Apps-Existing-SharePoint-2010-Server-Farm.aspx