The main difficulty in pronunciation of the Russian consonant [р] - [r] is to get the necessary vibration frequency of air stream. Often foreigners try to pronounce [р] with a roll - like "pppp" [rrrr] thinking that the Russian consonant [p] is characterized by its rolling sound. But it's not always sound this way. Moreover, not all Russians can pronounce it with a roll, e.g. the name "Гарри" [Garri] - Harry - where we should pronounce the sound [p] - [r] longer than we are used to. The Russian [p] - [r] is characterized by its hardness and clearness so you should work on it. A short staccato hard sound. That's why, if you can't pronounce the sound [p], start practise the sound [д] - [d] - then your tongue is at the right position, tip of the tongue touches teethridge. Such practice will help your speech apparatus to prepare for the required sound.
The best way to work on pronunciation of unfamiliar and difficult sounds is a practice. Start with pronunciation of staccato sounds [д] (д-д-д) [d-d-d), then add any vowel, for example [a] (да-да-да [da-da-da]). Try to return to the positioning of the sound [д] after [a] but pronounce the sound [p] - [r] (дар-дар-дар [dar-dar-dar]). Now skip the vowel to get the combination [др] - [dr]. Do this exercise again and again until you get the difference between the sounds [д] and [p].
Then add vowels to get the syllable. For example, "дра" [dra]. Make a whole word - "драма" [drama].
Драма [drama] - drama, дрова [drava] - firewood, друг [druk] - friend, дрожь [drozh] - tremble, здравствуйте [zdrastvuyte] - hello.
Трава [trava] - grass, трактор [traktar] - tractor, троллейбус [tral'eybus] - trolleybus.
Then train the words which begin with the sound [p] - [r]. It might seem to be more difficult task but the matter is to skip the sound-assistant [д] - [d] or [т] - [t].
Радость [radast'] - joy, раса [rasa] - race, радуга [raduga] - rainbow, рокот [rokat] - roar, Россия [Rasiya] - Russia, роль [rol'] - role, ручка [ruchka] - a pen, рыба [ryba] - fish.
The hard sound [p] - [r] has its pair - soft [p'] - [r']. To train this sound, you should use vowels which soften previous consonant (е [e], ё [yo], и [i], ю [yu], я [ya]) in phonetic exercises after the letter "P".
Дремота [drimota] - drowsiness, тренировка [trinirofka] - training, тряпка [tr'yapka] - rag, тревога [trivoga] - anxiety, редко [r'etka] - seldom, река [rika] - a river, риск [risk] - risk, ряса [r'yasa] - a frock.