Incorrect syllable stress and incorrect vowel sounds are two common reasons why beginning English speakers may not be easily understood by fluent English speakers. Make sure that you check and correct syllable stress and vowel sounds when you introduce new vocabulary words, so that initial errors don't become fossilized.
Syllable Stress
To make syllable stress visual, underline a stressed syllable or use syllable stress dots. I like syllable stress dots because they identify the total number of syllables in the word in addition to identifying the stressed syllables. Here's an example of a syllable stress matching activity that can be done on paper or turned into a digital matching game.
Vowel Sounds
Our six English vowels (A, E, I, O, U, Y) produce at least 16 different vowel sounds. To add complexity, the same vowel sound can be spelled in multiple ways. For example, the long-A sound can be spelled with A (at the end of a syllable), A+silent E, AI, AY, and EI.
To help learners identify the correct vowel sound and to make vowel sounds multisensory, I use keywords for each of the 16 vowel sounds that I teach.
For example, instead of referring to the "long-U" sound (which can also be spelled with O as in do, OE as in shoe, OO as in food, and OU as in Vancouver), I will refer to the "OO-like-Uber" sound and use a small picture of the Uber app as a visual reminder.
Once the learners are comfortable identifying the 16 vowel sounds, then I introduce the common spelling patterns for the most common vowel sounds:
Updated note: I started with the Color Vowel chart, but found the mnemonics to be challenging for very beginning learners. Then I moved to using only adjectives as key words (red for the short-E sound), but discovered that early literacy learners and learners with dyslexia had a very difficult time hearing the medial vowel sound. They heard the /r/ in red, but didn't associate the word with the short-E sound.
Finally I moved to using keywords where the target vowel sound is the first sound, wherever possible. This has made it much easier for the early literacy learners and learners with dyslexia to associate the keyword and the vowel sound. The keywords I'm currently using are
apple (short-A) and April (long-A)
exit (short-E) and eat (long-E)
itchy (short-I) and ice (long-I)
olive (short-O) and open (long-O)
up (short-U) and Uber (long-U)
arm (R-controlled A), orange (R-controlled O), and purple (ER, IR, UR)
good (short OO), oil (OI/OY), and ouch (OU/OW)
The main point is to use keywords that make sense for your learners.
Read my lips
I've found that it's harder for learners to hear pronunciation errors (such as /m/ vs /n/) in Zoom classes than in in-person classes. It can be very helpful for learners to imitate your mouth when making a specific vowel or consonant sound.
When demonstrating mouth position, use the Spotlight feature in Zoom to make sure all learners can see only your face. Encourage learners to use a small mirror to compare their mouth position with yours.
Here's a video of an instructor using a lip-reading activity in an in-person class to help learners focus on mouth position for the vocabulary words that they are practicing.