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For the weak R, on the other hand, which is probably what is giving you a hard time, and going back to your original question, a good way to fake it is to use the weak T or D sounds of English in words such as 'waTer' or 'laDDer'. I mean, in the General American pronunciation. It might be not exactly the same thing as the alveolar flap (weak R), but it's really close and you just need to speed it up a little bit to get there. I've had a good number of Chinese (Asian, for that matter) students and the weak R sound is one of the most difficult things for you guys to pick up (I don't know if it is THE most difficult one, because we have the nasal vowels to compete with it); you guys usually substitute an L sound for it, which is problematic, because 'calo' and 'caro' are different words. You can fake it till you learn it using the American English weak T or D as I said, but while you do it, try to feel and practice the correct sound.

You don't need to stress your tongue out in order to make it different from the L. The real goal here is to TEACH your BRAIN to recognize their difference. To do this, you need to understand what goes on in our mouths when we produce them: to make the L, I lift up the tip of my tongue, touch it on the roof of my mouth, and BLOCK the air from coming out through the middle or top of my mouth, instead allowing it to escape through the sides. I also can hold the L in my mouth and make it last as long as I want. To make the weak R, in turn, I quickly and gently bounce the tip of my tongue against the roof of my mouth, right behind my top teeth. I can't hold it; it's a one-movement sound. Try to do that. Slow it down. Feel it. Close your eyes. Over time, you'll not only be able to make it, but your brain will simultaneously understand it as a different sound altogether.

Hope this helps! ;)

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Phil is right. In Portuguese, we divide our R's into two different phonemes, called 'the weak R' and 'the strong R'. The weak R is the same as the weak R in Spanish (don't know if that's the terminology used in Spanish phonology) and technically is called an 'alveolar flap', represented by /ɾ/. The great majority of native speakers will indeed use an alveolar flap, but there is another possibility used by some people, especially in the countryside of some states here in Brazil or in relaxed speech, which is the 'retroflex flap', represented by [ɽ], produced with the tip of the tongue slightly curled up. The strong R, though, is the real treat. It can vary a LOT depending on the dialect. Usually it is some of the fricative sounds out there, that is, a sound produced by forcing air through a narrow space in your mouth. If you're interested in all the possibilities for our R's, just out of curiosity, let me refer you to two other answers I posted here on italki about that: https://www.italki.com/question/416366 and https://www.italki.com/discussion/159039

I don't speak any Chinese (though I'd love to someday!), but, according to Wikipedia, in Mandarin you guys do use the phoneme /x/, technically called a 'voiceless velar fricative', in words such as '河' ('river', right?). It seems that you can actually vary that sound to a [h], a 'voiceless glottal fricative', the same sound at the beginning of 'hat' in English. You also speak Cantonese, and, according to Wiktionary, this word here, https://en.wiktionary.org/wiki/%E5%93%88#Etymology_2, is also pronounced with a glottal fricative (again, the H in 'hat'). That is perfect. THIS is the sound you want for your strong R. If you use the velar one, you'll speak, as a rule, as someone more from Rio de Janeiro or other parts of the Southeast. If you use the H from 'hat', you'll be using the probably most common realization of that phoneme in Brazil. Just stick to that.