Saying highly (US): HIGH-lee. In HIGH-lee, the strong beat falls on the first syllable, and the stress mark /ˈ/ sits in front of the part of the word you say most strongly, so /ˈ/ would go before HIGH.
Breathe out softly for /h/ at the back of your mouth.
Open wide for /aɪ/, gliding from ah toward ee.
Let your tongue tip touch the ridge behind your teeth for /l/, then finish with a relaxed short /i/.
Keep /aɪ/ distinct from /eɪ/; highly does not start like hail. The sound pattern described here is the American one, so highly is being taught as said in American English: /ˈhaɪli/.
Try a phrase: a highly successful plan, keeping the ending light.
Hearing the last vowel
Compare the two vowel sounds with real words: highly starts like high, not like hail, which opens toward ay. Say each pair slowly, then at normal speed.
Check your ending by comparison: say happy, then highly. If the last syllable feels identical and easy, your weak vowel is working.
A note on the symbol /i/ without a length mark: in this transcription system it represents a weak vowel that may be sounded as /iː/, /ɪ/, or a sound between them. Dictionaries may show this final syllable differently, so trust a relaxed, short vowel over any single symbol.
Dictionaries may write the final weak vowel of highly differently, and that is fine. Keep the ending light and unstressed: a relaxed, short vowel matters more than any single symbol you see there.