Instructions:

• Throw an iron ingot, a lapis lazuli, and a fire charge into a crafting table to get a Portal Starter
• Fill your portal frame with crying obsidian and/or blocks of lapis lazuli (make sure that every block that you want to become portal has one or more faces open to air or water)
• Click on one of the blocks making up the core of the portal, and Voila!

Warnings:

This wasn't extensively stress tested, and the rules for portal creation are intentionally very lenient, so oddities are bound to happen, especially when two diagonal faces of a portal core block are open (yes, you can have frames that aren't completely filled in). Additionally, updating the faces of portal blocks won't have any affect on the portal, but updating the edges will almost always make the whole thing collapse.