Steps to create Application Pool
When you create a new application pool, you have the following two options:
1. You can either create a new application pool from scratch or
2. You can create a new application pool by importing the configuration
settings from an external XML file
How to create application pool
Below are the steps to create application pool from scratch.
1. Open IIS Manager.
2. right-click on the Application Pools node from the tree
view and select New->Application Pool from the context menu.

3. You will be presented with the following screen, where
you need to enter a name for the application pool.

4. You can see two options. Select the second option
"Use existing application pool as template" in the above screen. After you pick this option, the
“Application Pool name” dropdown box will be enabled.
Select DefaultAppPool from the dropdown box. As shown in below screen.

5. Click on the OK button.
6. After the pool is created, you can see the new application pool (TestApplicationPool)
under the Application Pools node. See the below screen.

7. Go to Web Sites > Default Web Site. And right-click on your web site.
Here I am taking the example of “test” web site. See the below screen shot.

8. Select Properties from the context menu. You will be presented with the following screen
9. In the Properties dialog box shown above, you can change the Application Pool
using the Application Pool dropdown option in the Directory tab. Because
we want our Web application to run under the new pool (TestApplicationPool),
select TestApplicationPool from the list.
10. Click the OK button.
11. Now go to your Application Pool (TestApplicationPool) again .
There you can see the name of your web site under your new application pool.
See the below screen.

12. Now restart the IIS.

Rollback Process
Below steps will help you to redoing the settings back to 1 default pool and delete the other application pools.
1. Go to Web Sites > Default Web Site. And right-click on your web site (test).
Here I am taking the example of “test” web site. See the below screen shot.

2. Select Properties from the context menu. You will be presented with the following screen.

3. In the Properties dialog box shown above, you can change the Application Pool
using the Application Pool dropdown option in the Directory tab. select DefaultAppPool from the list.
4. Click the OK button.
5. Now go to your Application Pool (TestApplicationPool) again .
You can see that there is no web site under your application pool(TestApplicationPool) .
Your web site(test) has moved to under DefaultAppPool. See the below screen shot.

6. Now there is no web site under your application pool, so you can delete the application pool
(TestApplicationPool).
7. Right-click on your Application Pool (TestApplicationPool)
and select Delete from the context menu. See the below screen shot.

8. You will be presented with the following screen, where you have to click the Yes button.
9. Now your application pool has deleted.
10. Restart the IIS as show above in one of the picture.