Multisyllabic words are words that contain more than one "beat" to the word. This video gives an overview on how to clap out syllables. Clapping out syllables reminds us to say every sound in the word.
https://www.youtube.com/watch?v=TT0_PI4AgyM
Syllable reduction is a phonological process, which means that instead of it being one single sound errors, like "w" for "r", it occurs in words as a pattern. So in the case of syllable reduction it happens in words with two or more syllables. The next video provides information on this process and strategies for addressing (visual cues).
https://www.youtube.com/watch?v=FfgdbHncHl4
For practicing these words feel free to visit any of the specific sounds in Articulation Nation and select "medial". All medial sounds in any of the targeted sound posts will have at least two syllables. You can practice this in words, phrases, and sentences. For more ideas (with an additional video or list of ways to work on this) visit the Speech and Language Kids site.