добрый день

I'm having difficulties grasping the pronunciation of the letters ы and щ, mainly because two different sources I'm using tell me to pronounce them differently. I was hoping I could get some clarification here.

My Russian course book describes ы as sounding like the i in English 'bit,' 'but with the tip of the tongue further back.' Not sure what to make of that.
The pronunciation book that came along with my Pimsleur CDs describes the sound as 'a quick eau-ee'
When listening to the Pimsleur CDs, I can only get the impression that ы sounds like the French word oui. Thus Вы would sound something like 'Vwi' in English. Is this correct?

As for щ, my grammar book describes the letter as ш being pronounced twice, as in the English words 'Welsh sheep'
But the Pimsleur book describes it as being 'a long and enunciated ш,' like when an English person holds a finger to their mouth and says 'sshhhhhh!' when they're trying to get someone to be quiet.

Which is correct?

Спасибо