Preface: After roughly 5 years of anguish, asperity, tribulation, wail, and woe; no resource helped my continual failure to pronounce [r] (the IPA symbol for alveolar trill). But today, I ail no more! Hallelujah!
An hour ago before writing this post, I fortuitously encountered this Quora answer by Clarissa Lohr that successfully guided me to pronounce [br]. Though I have yet to perfect [r] itself, I now need only to learn to drop the [b] in [br], to pronounce [r].
I emend it slightly to ameliorate readability, and to correct the IPA symbols used. For me, step 4 happened to be my first time and precise instant of pronouncing [r], to my eternal joy!
Here's how I learned it:
1. I would endlessly repeat the word [bəˈda].
The first sound should be labial, but it can be a voiced [b] or a voiceless, but very unaspirated [p], as you prefer.
The second sound should be short, unstressed and possibly easy to produce. I prefer Schwa (the vowel in about) since it's the least marked vowel for me, but if your native language doesn't have this sound, you may replace it by another vowel. [The IPA represents this sound, schwa, with [ə]] What matters is that this vowel comes naturally to you, and that you can pronounce it effortlessly.
The third sound should be a [d]
and the last sound should be a low vowel, i.e. [a] or something similar. This vowel should be stressed and you can lengthen it a bit if you like.
2. Then I would gradually shorten the first vowel, the Schwa. I tried to make it as short as I could. This took quite a while, so don't be impatient. I used to repeat the word for weeks, every time I was having a shower.
3. Finally I had shortened the vowel into non-existence. I was actually saying [bda]. The word had become monosyllabic.
4. At this point the [d] automatically changed into [ɾ̼]: the alveolar flap.
Then I somehow tried to hold the [ɾ̼] instead of immediately lowering my tongue to produce the [a]. It worked and the sound changed to [r].
5. Once I had mastered this I varied the phonological context for [r]. Non-coronal adjacent consonants are easier, so I first tried [kr]. [tr], [sr] etc. are a little trickier, and until this day I can't say [ʃr]. I also tried preconsonantal [r]. This was a little harder but finally I could say it.