How to pronounce "stupid", "YouTube" and words like this?

I assume that with stchupid you mean to represent a pronunciation starting with something like [stʃ] in the International phonetic alphabet (the "s" sound of "say" followed by the "ch" sound of "chew").

In British English, words that dictionaries show as having syllable-initial consonant clusters /tj/ and /dj/ are often pronounced with phonetic affricates [tʃ] or [dʒ]. This has been discussed elsewhere on this site; see Nardog's answer here. Likewise, /stj/ can sound like [stʃ].