We make the sound in "heed" (/i/) with our tongue very close to the top of our mouth. We also spread our lips so it looks like we are smiling.
You can see how Laurie looks like she is smiling when she says /i/ in the picture.
In the video clip of this same vowel sound, notice how the speaker looks like she is smiling when she says 'read.'
When we make the sound in "hid" (/I/), we don't look as much like we are smiling, and our tongues are lower in our mouths. Here is a picture of our mouths so that you can compare where the tongue is for these two sounds.
The sounds of heat and hit.