For this sound, the tongue is flat, and the corners of the lips are pulled to each side. You should be in a somewhat flat “smile” position. This is a long sound–speak it clearly and give it time if it is the stressed vowel in a word.
Use the video to listen and repeat common words.
appear, be, beauty, between, clear, complete, deep, east, eat, even, feel, feet, field, free, green, he, hear, heat, here, keep, leave, machine, me, near, need, people, piece, reach, read, real, see, seem, she, street, teach, these three, tree, we, week, wheel, year.