Pronounced as English “v”, Tolkien represented this sound as v at the beginning of words (mostly as the mutation of b or m) but as f at the end: “F represents f, except at the end of words, where it is used to represent the sound of v (as in English ‘of’): Nindalf, Fladrif (LotR/1113)”. Tolkien was inconsistent with how he represented this sound in the middle of words, however. Between vowels it was always v, but before consonants it was sometimes f as in S. Lefnui (LotR/750). As Tolkien described it in The Rivers and Beacon-hills of Gondor of the late 1960s:

Levnui is said to mean “fifth” (after Erui, Sirith, Serni, Morthond), but its form offers difficulties. It is spelt Lefnui on the Map; and that is preferable. For though in the Appendices f is said to have the sound of English f except when standing at the end of a word, voiceless f does not in fact occur medially before consonants (in uncompounded words or names) in Sindarin; while v is avoided before consonants in English (VT42/14).

In Noldorin of the 1930s and 40s, Tolkien generally used fn for [vn], but was less consistent in Sindarin, for example S. tavn “a thing made by handicraft” and the alternate spelling Levnui seen above. In both Noldorin and Sindarin Tolkien used vr rather than **fr pretty consistently. Tolkien’s statement that “f does not in fact occur medially before consonants” is not entirely correct, as there are words like niphred (technically an ancient compound). I personally prefer the spelling v in all non-final positions, as it makes the pronunciation more clear, and would adapt Noldorin words with fn to use vn in Neo-Sindarin.