"Action scenes. Dear gosh. I cannot for the life of me write an action scene. How do you do it?"
Great question. I learned mostly by doing it wrong for my first few manuscripts, so I'll save you the trouble of bad writing on yours.
7 Ingredients in an Action Scene
Use short sentences.
Keep them choppy. Keep them fast. This is action, folks, not verbose Austenesque description. Think about comic books. Bam! Wham! Kerplow! Not that you need excessive amounts of expletives. I'm just saying short sentences are like the shrapnel in a writer's arsenal.
Tug the heart strings.
Make the scene personal to the reader, yes, even if it's a velociraptor eating a protoceratops. How? Liken the protoceratops to a fuzzy, little kitten. That will hit home to every reader's heart -- even cat haters like me.
Keep it in real time.
This is not the time for backstory or too much internal monologue. Nouns and verbs, baby. Nouns and verbs. Keep the action popping like a bunch of popcorn kernels. Cut any unnecessary details.
Instinct over intellect.
This is also not the time to have your main character thinking lofty thoughts about what's going on. Keep it primal. Gut reactions. No philosophical rants.
It's got to be plausible.
Too many over-the-top stunts won't work. In order for a reader to believe in the scene, the action has to be credible. No one can fight a vampire, zombie, and Chuck Norris while suffering from the ravages of a hemorrhagic fever, no matter how awesome a hero they are.
The stakes have to be believable.
No hero is going to risk his life for a lollipop -- unless it's plutonium in disguise and the world will blow up if he doesn't. See what I did there? I went from unbelievable to believable. Do that. The stakes have got to justify the action.
Add in a time bomb.
Sure, you can use a real one, but a proverbial deadline works just as well. Your character needs to have a timeframe in which to vanquish whatever nemesis they're facing in order for the reader's heart rate to increase.
My biggest piece of advice is to write the scene as you see it in your head then edit it later. Get it down on paper first, then go over it with these ingredients in mind.