Because the word “do” is unstressed, it’s pronounced with the weak vowel /ə/.
You need a very relaxed mouth for this sound. Your mouth should be just slightly open, with your tongue relaxed, and neither front nor back. Just make a little noise with your voice.
The t of “what” is difficult to pronounce right before the d of “do”, so the t tends to disappear. “What do” becomes /ˈwɒdə/. “You” is also unstressed so can be /jə/ or /ju/.
“Reckon” simply means “think” in informal English, so the phrase is asking for an opinion: What do you think?