Put on some headphones. Block out everything. Don't even listen to music. Coffee shop ambience might help (it sometimes helps me), but nothing else.
Turn off your internet. Better yet, get away from wifi altogether so you won't be tempted to turn it back on. I use an app called Freedom that kills my ability to access the wifi for a scheduled period of time every day. This is because you might need to be bored to write. You know how when you're lying in the bed trying to sleep, but your mind just keeps coming up with shit? Or you're sitting on the toilet or taking a shower, and you start daydreaming? It's because your mind is unoccupied.
Remove all of the distractions, all of them. Shut the door and sit facing a corner. If you have to uninstall all the games and delete all the movies from your laptop to remove those distractions, do it if the writing is important.
What you're aiming to do is, like a hostage handcuffed to a radiator, you want to basically shackle yourself to your writing desk. You, your brain, that flashing son of a bitch cursor, and nothing else.
Then you picture the scene you want to write. Visualize it in your head, stare at the wall and picture it. Play it like a movie. Figure out where you want the scene to start--when Bob shoots Tim in the head? When Bob pulled out the gun? When Bob walked into the room? Do you want to go all the way back to when Bob left the house that morning?
When you figure that out, pick an action to start the text with. Bob takes the last sip of coffee and puts the mug on the table, picking up his keys. Bob slips into his sneakers and heads for the door. Bob locks the door behind him.
Then you decide what word you want to start that first sentence with. When? As? Bob? Locking? Locking the door, Bob dumped his keys into his jacket pocket and sauntered down the stepping-stones that leapfrogged across his unmowed grass.
Then you just keep going. And here's the magic part: don't give a fuck.
Write like you don't give a fuck. Because you shouldn't! Whatever you do, you can fix it later, like they say, you can "fix it in post". Go as long as you can without giving a fuck. Just work on pulling that story out of your head, work on unraveling the yarn of your mind until you're accomplished and sweaty and your back hurts and you're sitting there with a big floppy wad of word-yarn in your hands.