Antoine, I appreciate your question, but you might as well be saying, "I'm a very good cook, but I have no idea how to make separate meals."  Scenes are the heart of the film.  Without them, what do you got?  Credits.  (And that would be odd to see only that when you go to a movie to lose yourself in one heck of a good story.  "How was the movie?"  "Wonderful.  I loved all the names of people who did all this stuff that I never saw.")  Here's another way to put it:  Why do you watch a really good movie that you absolutely can't watch enough and you do so over and over again?  To get to that answer, here's a scenario:  You're just about to turn off the television (you haven't been outside for three weeks, a storm's on the way and you know you need to shovel the newspapers in front of your door) and on comes That Movie You Love and -- and here's the salient moment -- you just want to watch this one scene that you love.  This one scene.  Now, this-one-scene watching can often spread to watching-the-next-scene watching, and, before you know it, you've watched so many scenes that you're actually watching the rest of the movie again (the soggy newspapers be damned!).

If you did have time for only one scene (Say a cyclone was headed directly towards your house and you convinced yourself that it was important to take shelter -- although not until you watched that great scene where Al Pacino tells -- uh, you better get going.  Right.), you'd still be looking for some reward, some kind of emotional satisfaction, even from that one scene.  It would be like sitting down for a very, very (very) short movie.  But sit we do.  Thousands of us.  Maybe millions.  Myself included (I've been late for meetings -- I'm not boasting here -- because I found myself saying, as many of have said so many times, "I love this part.")  That "part" is most likely a scene or a part of a scene from a film somebody cherishes.  If you think about it, you usually watch that part, that scene, until it concludes, needing that mini-emotional catharsis before heading off into your real "unmovie" life (where there are real consequences for being late after staying too long, watching movies, which makes you late.  Sort of existential and Sartre-esque, wouldn't you agree, mon ami?)

That's a good place to begin in order to write good scenes:  recognize their importance.  True, each scene is supposed to inform the overall story and provide information, revealing the journey of the protagonist, and many more tasks that you can find out about in any screenwriting primer.  Those technical qualities are all good to keep in mind, but I encourage writers to go much deeper than the conscious foundation of story and plumb the depths of their own, unique and individual recesses of their creative souls.  Deep, huh?  And one way to do that in terms of scene writing is to initially take out all the stops and (and I know this may sound simplistic at first) let yourself write.  Give yourself permission to write the scenes that you've always wanted to write, that nobody has before, and those that YOU would like to see up there on the screen.  

You can think of your scenes as mini-movies.  Give them purpose, flair, emotion, conflict (ah, conflict.  Have you noticed how effective scenes usually have conflict, either overt or covert?), action, tension, relief of tension and then more tension.  Twist us in our seats and call us "pretzel people."  Make us beg for release and don't give it to us until we've earned it (whatever that means).  Surprise us; shock us; electrocute -- I mean "electrify" -- us; agonize us; soothe us and confuse us (maybe not in that order).  If the director is going to do another setup and the crew is going to light it (or not light it if it's outside.  But even Apollo up there doesn't want to waste his time making sure the sun is in the right position in the sky if he isn't going to enjoy the spectacle down below.), then you better give them something that will make it worth everybody's while (including the investors.  Those are the creepy people who sneer at people taking long lunches and look at their watches every few minutes.  Not really.  Some of my best friends are investors.  Not really.  Which might account for why I haven't sold a screenplay recently.)

What I'm trying to get across to you is that your scenes aren't just bridges and connecting points to the final destination of the screenplay.  They ARE the screenplay.  Maybe I should come up with some pithy saying like, "The screenplay is the scenes," -- grammatically incorrect, yet so provocative in its stark rebelliousness -- (wherein I'll catch the conscience of the... producer.  Okay, I never said I was another Shakespeare.) that will resound throughout literary history as a watermark of screenplay wisdom.  How about that?!  I could -- "watermark"!  I forgot about those newspapers!

DcH