I've been asked to write about this, and at first I didn't even know where to begin, never having given much thought about this particular topic when I'm writing. And then I thought, no, not true, I have, it's just that now it seems to flow and I forgot to think about it, but I did think about it when I first started, and I hated how my dialogue was clumsy, how it seemed like I jumped from people talking to describing the setting, to wondering when it was appropriate again to jump to people talking. You get the drift. So how come I don't think about it anymore? Because I seem to have found a certain rhythm. I think. Let me share what works for me, and hopefully it will help you too, because for me writing started having a certain beat, a pulse, almost, like in music. 

Listen for a tempo. This is probably the best I can describe it. It all comes down to listening for a tempo in your own writing and then comparing it with the tempo of the books you are reading. So, at first, don't worry about transitions at all, simply write, but after you are done, every day, read. Read some really good stuff, read books that you love. The next day write again, and read again. Keep doing this and allow yourself to be horrible, be okay with sounding very choppy or unnatural in your writing. The most important thing is to keep writing and reading every day, even if it's only 30 minutes each. Soon you will start seeing patterns, you will notice how some authors rely heavily on dialogue, while others hardly use it at all (just read Nabokov's Lolita, you'll see what I mean). Please, don't throw rotten tomatoes at me here, but my prediction is, it will take you at least 1 year to get it, you will start seeing distinctive patterns. It will take you another 4 years to get really good at it, if you are writing full time. I'm not there yet, and it's not me who calculated it. Malcolm Gladwell said in his book Outliers that it takes 10K hours to become good at anything. So, the main thing you can do for your writing to flow smoothly is simply to... keep writing and reading!

Measure it in paragraphs.  Until I developed a certain tempo, which my beta readers told me I have (I lean more heavily on descriptions rather than dialogue), I used the rule of paragraphs. I didn't read about it in a book, it was simply easy to visually remember and I have glimpsed it in other books too. So, it went like this. I would always open each chapter with one paragraph of description, no more (still do), and then would open the next paragraph with dialogue, go for a while, then do a paragraph of description again. I did it like in a song, alternating between the main verse and the chorus, if you will. It wasn't beautiful, but at least it was organized and it kept me on track. And that's all a beginning rookie writer like me needs, some kind of organizational system to hold on to when everything else seems to be falling apart.

Highlight important stuff for the story. Another way of thinking about transitions is to only highlight what the reader absolutely needs to know and not mention the rest. For example, if your main character is wearing a pink panda costume, make sure you describe said costume in minute detail, because it's an out of ordinary thing to do. If, on the other hand, your character buys a pink panda costume every day in a town where everyone wears pink panda costumes, then it's not something out of the ordinary, and you only need to mention the pink panda costume once in a sentence, no more. This is where the idea of the rhythm comes in. You write about unusual interesting stuff that is new, and you leave out the boring stuff. Now, in my experience, this really comes together in the rewrites, when you start seeing what is fluff and what is solid. Until then, especially in your first draft, don't worry about being choppy. The goal is not to be perfect, the goal is to get the story down on paper.

Read it aloud. This is the simplest exercise you can do for your transitions. Read your own writing yourself aloud, and, if at all possible, ask a friend to read to you several pages of your writing. Where you will stutter, where your friend will stutter, that's where it's rough and you have to smooth it out.  How? Simple. Cut to the next piece of action. The most important thing for your story is to make the reader turn the pages, and you have to ask yourself a question. That line of description, will it make my reader want to know more? That line of dialogue, will it make my reader sit on the edge of the seat, dying to know what happens next? Yes? Great! No? Cut it. The more you do this, the more you will start feeling a certain tempo that is unique to your own writing. It might be fast and choppy, or it might be slow and lyrical. Whatever it is, without doing a lot of it you won't find it, and once you do find it, you will feel it, and you writing will star flowing smoothly. It's the best feeling in the world.

I honestly hope I wrote here something that made sense to you and that you could put to practical use, because never specifically studied transitions, I don't even know if I'm supposed to use some big important words that people use when they talk about transitions. All I do is feel, and transitions in writing are like transitions in music, they are fluid, they are hard to catch and put in a box, but if you really listen for them, listen for them in your own writing and in the writing of others, one day a veil will fall off your eyes and you will see them. And, once you do, your own writing will start singing in accordance to that certain rhythm that's yours and yours alone.