The 'ee' as in 'she' vowel. To make the sound, the front part of the tongue raises very high towards the roof of the mouth while the tip still remains down, lightly touching behind the bottom front teeth, ee, ee, ee. The tongue doesn't touch the roof of the mouth, but it's very close up there, ee, ee. You can see also the corners of the lips pull wide, ee, ee.
Here is a photo of the 'ee' as in 'she' vowel sound on the right, compared with the mouth at rest on the left. Notice that the corners of the mouth are pulled slightly back in this ee sound. And here with the inside parts drawn in. As with all vowels, the soft palate is raised, but more importantly, note the tongue. It's raised very high in the mid-front, though not quite touching the roof of the mouth. The tip of the tongue lightly touches the back of the bottom front teeth. Sample words: please, ski, even. Sample sentence: She needs to see there's a reason to believe me. Now you'll see this sentence close up and in slow motion, both straight on and from an angle, so you can really study how the mouth moves when making this sound.
She, S-H sh, and the ee. Needs, see how much further the corners of the lips pull back, needs, as it is more stressed than she. To see, again, the corners of the mouth are pulled back. There's, the tongue comes through the teeth, a reason, again the 'ee' as in 'she'" rr ee son. To believe, lip comes up to make the V, me.
And now from the angle. She, sh here, she, needs, tongue goes up to make the N. And watch the corners of the mouth really pull back. Needs. To, t-uh here. See, teeth come together to make the S, see. There's, tongue comes through the teeth. A reason, lips form the R, re-, corners pull back, re-son. To believe. Lips together for the B, tongue up to make the L and the corners of the lips pull back for the ee. Lip moves up to make the V. Me, corners pull back again for the ee sound. That's it, and thanks so much for using Rachel's English.