In American English, is is normally unstressed and uses the short vowel /ɪ/, the vowel of sit. Dictionaries often write it as /ɪz/.
As the third-person singular of be, is links a subject to a description or place: She is a teacher. The gym is open.
Mouth movements for the vowel
Open your mouth a little and relax your lips. The vowel /ɪ/, as in sit and gym, is slightly lower and more retracted than the tense vowel of seat, so keep the tongue slightly lower than a stretched ee sound.
Let the tongue rest near the front of the mouth without tension. Say sit first, then hold that vowel and add a /z/ hum: the vocal cords vibrate while the tongue tip stays close, not touching, just behind the teeth.
Keep the whole word light and quick. In normal speech 'is' is unstressed, so let it take less force than the words around it. The ending /z/ is a soft buzz, closer to his than the sharp /s/ in hiss.
Say it in a phrase
Try the short phrase This is my seat. Say this and is lightly, then let the stress land on seat.
Keep the contrast in mind: is ends with a buzzing /z/ sound, while ice ends with a sharp /s/ with no voice. The gift is nice versus I like ice shows the difference.
A natural question works too: Where is the gym? Let is stay soft and quick between the other words.
For paired practice, use Sit down; it is in here. The vowel in is matches the vowel in sit, so alternate them inside the sentence: sit, is, sit, is.