The letter a: about, banana, was
The letter e: the, open, travel, item
The letter i: animal, family
The letter o: above, gallon, dinosaur, another
The letter u: particular
The schwa sound often times happens in an unstressed syllable. I think this is too complicated of a rule to teach to a beginning reader. It may be best just to introduce the sound as one to try when a long or short vowel sound doesn't work.
Here's a fun video about the schwa sound.
So . . .
When a long or short vowel sound doesn't work, try the schwa sound. It's like the short u sound, or the sound a person makes when he or she is thinking.
Learning vowel sounds and sounds for vowel combinations may be the hardest part of learning to read. I think teaching the schwa sound and not calling all words with the schwa sound a sight word greatly improves a new reader's chances of correctly sounding out many more words.Here are a few links you may find helpful to learn more about the schwa sound.
Teaching the Schwa from BBC
Find more from me at Parent and Child Reading Assistance.