'th' in special cases

I wonder how to pronounce 'ch', 'sh' before the, like watch the movie, wash the clothes. coz I find it hard to pronounce both quickly and correctly

In careful speech, these are pronounced as written, but when slurred, the "ch" in "watch" can sound like "sh".

I don't see any problem with "wash the clothes", however.

Just don't speak fast. In fact, if you use words like "coz", don't English at all.