How do you make an oblivion portal in oblivion

I've played this game through like 5 times on each difficulty on the xbox360 and there is no real way you can without mods.

If you play along the storyline eventually they pop up and there are 16 gates total. One closed they cant be repopened so make sure you search everywhere in that oblivion portal before you close it.

there are 100 gates in the game, 16 are part of the main quest
the rest open at random when you get near them