The rule is simple: If the last vowel is one of “a, ı, o, u” the suffix should be –lar; if the vowel is one of “e, i, ö, ü” the suffix should be –ler.
Yeah, I should have read the starting description more closely! Now it's just a matter of remembering which one is which!
Don't know if you're still learning Turkish, but in the beginning, which was just three months ago, I relied quite heavily upon this site here:
whenever I wasn't sure about what vowel should be used for the suffix.
On this page you will find a section called "Noun declensions by example." It provides declensions though the cases for 10 words (house, coffee, front, roof, ceiling, handle, ball, neighbor, dog, and book), which is enough to cover just about any word you will encounter in Turkish.
I relied upon it so heavily in the beginning that I began to think I'd never have to stop referring to it, but eventually your mind, given enough time and practice, will synthesize this rule of vowel harmony.
Schatzie's advice in this thread is really quite good, but if you're more of a visual learner than an audial or kinetic learner, this web page may be a better reference for you in the beginning.
Hope that helps some of you out there.
As the first comment says, "elma"has "a" as its last vowel. So it gets the suffix "lar".
If the tongue is already positioned to form vowels in the front of the mouth, use "ler." If the tongue is already positioned to form vowels in the back of the mouth, use "lar." This rule directly applies to the last vowel in the root word. Now look at the examples in the phrase above, and it should make sense. Think about how your tongue moves as you practice saying "ler" and "lar."