You'll have to find an IR blaster IC that accepts some sort of communication bus that's available in your phone and converts it to IR communication (I'd just try to find what's already used in phones with a IR blaster included). Then you'd have to find a way to wire that IC and other necessary components (capacitors, resistors, etc) onto that bus and get it powered in your phone. Then you'd have to find a way to fit that group of components in your phone. Then you'd have to find a IR LED that will fit in your phone and find a way to wire it to your IC. Then you'd have to modifty the phone case to allow that IR LED to transmit out. Then you'd have to find (or make) drivers for the IC you found, build it into the version of Android you want to use. Hopefully at that point you can find an app that is able to access that driver and use the blaster.
If this is a project you have control of (i.e. it's something you're developing yourself) then it might not be all that bad. The software part may be the worst part if drivers aren't available. If you're talking about adding it to an existing device (like a mass produced Android phone you own) then it will probably be anywhere from very difficult to impossible, especially if you want to keep the same enclosure.
Either way, you'll have to be way more specific and give many more details if you want anything more specific than that.