1. Make sure you have
Lucky Patcher installed.
2. Start
Lucky Patcher and hit the "Switches" button on the menu bar. There should be an option for "Google Billing Emulation" and "Proxy Server for emulation of inapp purchases" make sure they are both on.
3. Under the "toolbox" menu option, choose "Patch to android", and choose "Signature Verification status always true", "Disable .apk Signature Verification", and "Disable signature verification in the package manager"
With those set properly, when you try to make an in-app purchase, an LP window will pop up asking if you want LP to handle the purchase emulation. Hit yes, get your premium stuff.