I find it helps to work backwards with clustered consonants so get "tsete" down first and then but "ch" in front.
Have a look at the Tips & Notes on pronunciation here: https://www.duolingo.com/skill/cs/Phrases/tips-and-notes. If you're already familiar with the basic sounds, repeated listening, followed by repeated speaking of a word, should help. Maybe one of the native Czech speakers (on the team, or following the course) can offer more specific advice.
Yes, this has been working for me to just stop and repeat it at full speed and/or slow speed until I can say the full phrase perfectly as fast as the native speaker. However, I must admit this is a booger of a word.
chcete phonetically seems it could be: ph-se-te, or ... s-ts-te, or ... sh-ts-te, or ... het-se-te
Based on the following, I'm going with het-se-te"
"ch is considered a single letter in Czech and is found after h in dictionaries. It is the UNVOICED COUNTERPART TO to h, a sound which no longer exists in most dialects of English. Scots has retained it, for example in Loch Ness." (It is pronounced in the front of the mouth where the 'k' is pronounced, hence is why that 'ch' was used in words like Loch Ness, which sounds like Lok Ness.)
"c represents a sound which doesn't quite exist in English, but it is very close to the ts as the end of cats. It is, however, a single sound rather than a combination of two."
So first the ch sound or "he" and then the "tse" which run together sounds like hetsete. There is no 's' or 'ph' or 'sh' sound at the beginning or where would it come from! Hope that helps.
This is the formal Vy used for persons you may not address Ty. The T-V distinction https://en.wikipedia.org/wiki/T%E2%80%93V_distinction
It was also mentioned in some previous Tips and notes in more detail. Check the Thou skill.