Quick warping is a way to shorten the animation on the warps between the Light and Dark Worlds. It saves 34 frames per warp, which adds up to about 12.5 seconds over a full Any% NMG run.
How to perform it
The only factor on whether a warp will have a quick or a slow animation, is the horizontal position of the camera (in other words, the X coordinate of the left and right edge of your screen). For every eight pixels, there will be two that ends up being quick warps. Note that the Y coordinate (top/bottom of your screen) does not matter at all.
Since the camera follows Link around, you can usually use Link as a cue for this, but it's important to understand the distinction, because in certain screens, the camera can sometimes be offset by a pixel or more, making your Link cues not work.
Additionally, if you are near the left or right edge of a screen on the overworld, the camera may lock and quit scrolling left and right. While in these positions, quick warps are not possible. No such situation exists in an Any% NMG run, but in other runs, such as 100%, a quick warp may not always be possible due to camera locks.
You can find basic info for all the quick warps here: 
Flute Quick Warp
This is a screen where your camera can get offset sometimes. It happens during your interaction with the Flute Boy - the camera sometimes moves before Link starts moving, depending on what you do with your dpad, and how you initiate the conversation. By holdingduring the entire conversation, you seem to always end up not being offset, but that also means you're not mashing your dpad on the last text box.
To help with this quick warp, I recommend using your menu to the mirror as a buffer to easily see where you are and how to adjust. If you are a god though, you can switch to the mirror prior to getting the shovel to save a couple of frames on that menu, and then get the quick warp bufferless.
The general QW area from right to left:
This is a bad pixel to the right of where you need to be (XP: 142)
I like to look at the tree to the right of Link as a cue here. There's a 1 pixel gap between the right root of the tree and the edge of the screen. Pretty easy to recognize.
Here the root is perfectly aligned with the edge of the screen. You can also still see a strip of grass above the root between the edge of the screen and the leaves in the tree. This 1 of your 2 good pixels that will get the quick warp.
Here the black border of the root is cut off some, but you still have a 1px gap of grass right above it. This is your 2nd good pixel that will quick warp.
Now you can't see any grass above the root at the right edge of the screen. This is a bad pixel that won't warp you.