Snailplane's advice of listening to minimal pairings of words which contain the two sounds is excellent. Being able to hear the difference is the first step in learning to say the two sounds distinctly, as you'll be able to tell which one you're saying and can correct yourself when wrong. Listen to the recordings a few times, then follow along and try your best to imitate the sounds, even if you can't get it right.
To start learning how to make the sounds properly, get a native speaker to make the sounds for you, slowly and with exaggerated enunciation. If you can't get a native speaker, find a fluent one, but because your native tongue doesn't distinguish the two sounds, someone who speaks Russian natively might not pronounce the English sounds well enough. If you can't get a person then find some videos, but they are far less useful for this. Watch the way their mouth moves, and ask them to describe the position of their lips and tongue, as well as where and how the air moves in their throat and mouth.
These two sounds are very similar; no doubt that's why you're having such difficulty. Both are aspirated vowels. The shape of the air exiting your mouth will make the distinction between the two sounds. In my speech, the main difference is that when saying /ɛ/, the tip of my tongue touches the top half of the back of my bottom teeth; when saying /æ/, my tongue sits more or less level in the middle of my mouth and doesn't touch my teeth. Also, when I say /æ/, my mouth opens wider than with /ɛ/.
Placing my hand in front of my mouth, I can feel a difference in where the bulk of the air exits my mouth. Air from /æ/ comes out much higher, from the middle and top, than with /ɛ/, which comes out the bottom. This is because of the shape of my tongue. When saying /ɛ/, my tongue is curved downward from the middle to the tip. With /æ/, there's very little curvature.