For both /aɪ/ and /aʊ/ you begin with an open, mid-front, unrounded vowel.
GA /aɪ/ ends in a centralized, half-close, front vowel.
GA /aʊ/ ends in a centralized, half-close, rounded, back vowel.
Advice for Dutch learners:
The problem for Dutch students is that they usually don’t begin with the first vowel open enough (as is the problem with /ɑː/ and /æ/ as well).
Some words containing /aɪ/:
• island, sign, bicycle’
• side, quite, guide
• my, cycle
• high, night
• lie, tie
Some words containing /aʊ/:
• how, shower, towel, crowd, sow
• out, shout, pronounce, fountain, announce, boundary
In Southern American dialects /aɪ/ is often monophthongized to /aː/, i.e. there is no centering glide. In some dialects /aʊ/ is fronted, so that the words now and loud would be pronounced: [næʊ] and [læʊd], with the tongue positioned as for /æ/.