Simply put, a Korean 시조/shijo is alot like a Japanese haiku: rhyming doesn't matter and its all in the syllables. I spell it shijo as it most closely matches the Korean pronunciation of shijo as opposed to the substandard sijo as sijo could be erroneously pronounced as see joe. Sijo? No. I see Dick and Jane.
A shijo is generally a three line poem with a pause in each of the three lines. The pause is not necessary but is found in some Korean shijo poems.
Like haiku, rhyming is not necessary.
Some may tell you that a shijo is not three lines but that it is more. They are referring to what may be called a shijo sequence. But lets not fill the air with noise and just stick to a shijo proper for now.
Many of these short three line shijos have been written as far back as the Goguryeo and Paekjae kingdoms. Many have been written during the Koryo and Joseon Dynasties.
Now for the syllables.
Generally a shijo has the first line 15 syllables, the next 15 and the final line with either 14 or 17 syllables.
Naver will back me up on that as well as one of my books on shijo. Thus, this goes for a traditional shijo.
You will notice that most haiku follow a 17 syllable total with lines being 5,7,5 syllables pattern. I say most. There are many in the world haiku association who deviate from this and still call their poems haiku. You need only to look to the Internet for examples illustrating this mass proliferation of haiku poems.
The proliferation of syllable patterns with shijo runs about the same as with haiku.
In my book which contains mostly choseon era shijo in Korean with English translations contains many syllable pattern deviations in the original korean.
Thus you may write in any of the line/syllable patterns below at your leisure (as how the best poetry is written for it is in leisure that the mischevious muse seems to find us best). These were the patterns found in my book:
Many in the traditional 15, 15, 14. Some in the 15, 15, 17. Some 14, 14, 15
Many with 15, 14, 15. A few were 14, 15, 15.
I found a few that were 15, 15, 16 and two, one was 13, 15, 16 and another 16, 14, 15.
So you can see they are generally with 15 and 14 syllables.
Here's one I'd written while waiting for my wife to come home:
Fukushima reactor radiating the countryside
Korean people showing only the slightest of concern
amid the busy bustle of people, life carries on.
See! 15, 15 14. A pitiful shijo. I even used the word people twice! Now you know enough. Go forth and kick my ass. Later we'll get into shijo sequences. MWT.