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If you use computer:
Download the ipa file to computer and copy the file to Installous in your iPhone.
You should copy the file under /User/Documents/Installous/Downloads.
After copy the file, open Installous select Downloads and select the file, Installous will install the app for you.

If not using computer:
You need to use an app called Download Pro to download the ipa file directly to your iPhone.
After the file is downloaded , you have to find out where is the file located, use iFile to copy the file to the above directory and you can install the app same as above.

How to find out where is the downloaded file:
You need iFunbox (in your computer) or OpenSSH (in your computer and your iPhone), these software like file explorer, you can see all your files in your iPhone.
Use iFunbox or OpenSSH to search the downloaded file and find out which directory is under.
Use iFile (you can download this app from Cydia) (this is also same as file explorer, you can view all files and directories in your iPhone), locate the file and copy it to Installous, and follow the step as above.