oooo this is hard to explain through writing, but let’s use “나를” as an example to make it easier. as you probably know, it is romanized as na-reul. when you prounce “나,” the edge of the tongue should be hitting near your upper teeth. When you pronounce the first “ㄹ” in “나를” your tongue should move to the top of your mouth and after you finish saying it you bring your tongue down for the L sound. my best advice for learning how to pronounce it correctly is either watching native korean speakers use it or listen to songs/podcasts so you can understand how to pronounce it naturally. hope this helps!

http://m.korean.go.kr/ this site is one of official korean language sites. in the menu, u can find 국어.한국어교육 menu. and there are some side menu. one of them is 바른소리. then u can learn the pronunciation for each letter even intonations. i hope enjoy it. ^^

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).

https://www.youtube.com/watch?v=rKMeaF566xc and there are several other videos linked there with more explanations.