One important thing about 를 is that it is not something used generally coming at any position in a word.
를 almost always comes after a vowel-ending syllable (나를, 걔를, etc), so there's no point in knowing how it's pronounced in isolation. This is why we have both 을 (after a consonant) and 를 (after a vowel) for the same function.
(FYI, Korean traditionally didn't allow ㄹ at the beginning of a word at all, but this rule changed about a hundred years ago because of the introduction of foreign-originated words starting with an r or l, such as 라디오 and 러시아)
As was mentioned by Gabi, 를 sounds like "reul" after a vowel, with the first ㄹ like an r and the second an l.
So think of "Carl", or 카를 if transliterated to emphasize the trailing 를 sound.
를 is like the back part of "Carl", except that the r sound is blunt and explicit unlike the smooth and light English r.
You can think of "Carlos" (카를로스) as it is pronounced in Spanish. Take off the beginning "Ca" and ending "(l)os" part, then you're left with the 를 sound (but try not to roll the r like they do).