This is my notes from the information found on 123teachme. There they also list example words with sound files.
A diphthong1 is formed in Spanish when the unaccented vowel i or u proceeds or follows another vowel, or a y follows a vowel. Attaching an i adds a "y" sound, attaching u adds a "w" sound.2
No diphthong is formed during a combination of two of the following vowels: a, e, o.
- two vowels glided together and pronounced within the same syllable [↩]
- The Spanish y is said, i believe, with putting the tip of your tongue towards the bottom of your mouth by the gum of your bottom teeth. This is in contrast to English where the tip stays neutral or even goes up towards the roof of the mouth when making the "y" sound. [↩]