Spelled under, pronounced /ˈʌn.dər/.
The first syllable carries the stress, and under /ˈʌn.dər/ is the standard spelling of this common preposition.
The letter-to-sound mapping is simple: u = /ʌ/, the short vowel of cup, and n, d, r = /n/, /d/, /r/.
The second syllable is the trap. Its e looks like /e/ as in bed, but the unstressed syllable reduces the vowel to /ə/ (or /ər/ in rhotic accents), matching the /dər/ shown in the transcription.
Two syllables only, stress on the first: UN-der, never un-der-EE. Keep the ending light and unstressed.
The cat sat under the table. A simple everyday picture: the cat is on the floor, right beneath the table.
Put it under the bed so nobody sees it. A natural everyday use, easy to say once you land the stress on the first syllable.