Well in Generation II, what you had to do was find the 'nibbles' of each stat's IV, except for the HP IV.
Using Bulbapedia's example:
Attack IV: 10 - Nibble:
Defense IV: 9 - Nibble:
Speed IV: 1 - Nibble:
Special IV: 14 - Nibble:
(Okay, I have no idea how to calculate nibbles, and in all due honestly, I don't care. So please don't ask me about it, but rather take the tough challenge of looking it up yourself. It's a binary thing. D:)
Now, you have to take the two middle digits of the nibble, and stick them all into a new number, in the respective order below.
Attack Nibble: 1---01---0
Defense Nibble: 1---00---1
Speed Nibble: 0---00---1
Special Nibble: 1---11---0
Now, the combination of numbers is now a nibble itself, of another number. In this case, it's 67.
And now, you divide this number by ten, and round it down.
67 / 10
And correspond this letter to it's letter in the alphabet. You know 1=A, 2=B etc. In this case, 6=G. So it will be Unown G!
So, there you have it. However, in Gen III it was different. It was calculated by personality values. There are many possible personality values, and I can't be bothered talking about it, so if you wanna look just click here and scroll down to 'Unown's Letter'.
Generation IV onwards Unown's form was calculated with a separate form generator.
Hope I helped. :)