If you're looking for the underlying form, it's often just marked as /r/ for simplicity (which also corresponds to a previous realisation by chance, still found in some regions -- see, for example, Laurentian French, though it's being lost).* If you want the way it's realised, then it's variable both regionally (ex:
http://www.hum.uu.nl/medewerkers/h.vandevelde/onderzoek/Demolin.pdf) and even within speakers (ex:
http://www.ling.upenn.edu/~gillian/PAPERS/LifeChangeR.pdf), with some of the main options being: uvular trills or fricatives and alveolar trills. Phonologically, it's worth noting that it patterns with the voiced fricatives in the case of triggering lengthening (V --> V: / _ {v z ʒ r}), but that it does behave slightly differently in other respects (ability to be deleted, availability in clusters, devoicing (mostly in a cluster with voiceless obstruents)).
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* Less often, but still possible, the capital /R/ is used to show an abstraction, but, like in [R], will often refer to the uvular trill pronunciation for ease of typing.
