How to install .ipa files using iFile & Installous on Jailbroken iOS

Requirements:

• Jailbroken iDevice
• Installous
• Safari Download Manager ( Note: Not Compatible with iOS 5)
• iFile

Method:
The idea is to download the file (.ipa) by your iDevice Safari (using Safari Download Manager), then to transfer the file (.ipa) to Installous downloads folder, then you can install it from Installous

1. Start Safari, go to the webpage which contains the wanted game/app (.ipa) and start the download
2. When finished >> Open in iFile
3. iFile will take you to the needed folder = /User/Documents >> Click right upper tab (edit) >> Tick the downloaded file >> Click right lower tab (cut/copy)>> Click Cut >> Click right upper tab (Done)
4. Open Installous folder (from the same window) >> open Downloads folder inside Installous = /User/Documents/Installous/Downloads
5. Inside Downloads folder >> Click right upper tab (edit) >> Click right lower tab (paste) >> Click Paste>> Click right upper tab (Done)
6. Run Installous now >> Click Downloads >> You will find your file (without thumbnail) >> Click your file >> Install >> You Are Done

Source: http://ipad-os.net/ifile-cydia-app-for-iphoneipodipad-freedebrepo/