You might find it helpful to see if you have this vowel in your native language. Check the chart on this wikipedia link.
Common error patterns for this vowel are the tongue being too high (sounds like/ɛ/) or the tongue being too far back (sounds like /ɑ/). So "had" would sound like "head" (tongue too high) or "bad" would sound like "bod" (tongue too far back).
Practice this vowel on some common words:
Now, try some repeated phrases:
that man, that time, that thing, that email, that day
have a friend, have a meeting, have a lunch, have a few things
Check your vowel with some minimal pairs: