You may read several explanations (as I did) but the closest I got to understand is that: щ is pronunced as English "sh", as in "she". But ш is pronounced a little different, placing the tongue kind of rolled up backwards and then making the same sound as a "sh". That I the closest I have got and most of the time, Russians tell me I am pronouncing correctly. I hope this helps you.

Щавель, щука, борщ, Шуруп, шорох, шпатель Щ is more stronger then Ш

As far as I know, the Russian words with the letter щ have the English spelling shch, e.g. shchi-щи (the Russian cabbage soup). However, the sound [щ ] is quite different. I would say it's like the difference between the pronunciation of two English words shorts (similar to the Russian sound [ш] [шoтc]) and shirts (similar to the Russian sound [щ] [щ'oтc]).The sound [ш] with any vowel that goes after it is always hard.The sound [щ] with any vowel that goes after it is always soft. We also use the sound [щ] in the words beginning with -сч, e.g. "счастье"(happiness), "счёт"(score). We pronounce [сч] not as two sounds [s-ch], but as one sound [щ] :[щаст'e], [щ'oт]. Hope, it will help.

ш is a hard consonant, щ is a soft consonant. soft consonants require the tip of your tongue to be down 'under'/'behind' your bottom teeth, while the middle of your tongue (spine) is up close to alveoles. hard ш requires the tip of your tongue to be close to alveoles. basically, it's the only difference you need to care of to make these sounds distinct. the rest is the same for both of the sounds - pressing the flat tongue to your back teeth and making a hissing sound.