The easiest way to Install .IPA of iPhone Apps is with a jailbroken device and app sync however, for those without Jailbreak here is a simple step by step guide.
1- Download the .IPA file(s) that you want to install on your device
2- Download the iResign zip file
3- Get your UDID from here and Register your UDID
4- Launch iResign tool and browse for the downloaded .IPA file of the app you want to install
5- In the second field make sure you have Keychain Access for your certificate.
e.g: iPhone Developer: Fred Smith (I9Bxxxxxxx)
6- Finally click on ReSign and wait for the tool to complete.
7- Once finished the IPA file of the app is now re-signed.
8- Finally simply drag the resigned copy of the iPA into iTunes
9- Then now Sync your iPhone to install new app on your device
We hope you find this guide useful.