How can I practice differentiating between the /æ/ and /ɛ/ sounds in English phonology?

Here is a typical English vowel chart:

As you can see, /æ/ and /ɛ/ are quite close to each other, and so have quite a similar sound. The difference is in the degree of openness: /æ/ is “near-open” and /ɛ/ is “open-mid”. If you want to make a clear distinction between the two, you need to practice, practice, practice. When I was learning phonetics, I found the best practice was to try to make long continuous vowels that go along the axes, like /iiiiiiiieeeeeeeeɛɛɛɛɛɛɛɛææææææææaaaaaaaa/. Once I was able to master making the entire continuum, it became easier to find individual points along the continuum.

Celery e as in evet Salary as the e in lütfen

You could pronounce salary just as if it were a Turkish word as well.