As described by "X" from instructions by BS

There are many ways to turn a flake into the negative, (and back to positive), but the following are EASILY the simplest and most "fool proof" that I have encountered thus far.

If you are a beginner, go try it out.  If you already know how to flip a flake into negative but are using the old "take a chunk out" method to do so, I HIGHLY recommend changing to this much easier, Scissor Police approved method!

Many ThanX to BS for sharing these "Easiest of All" steps for making negatives & positives! ThanX also for the diagrams :D  - × -
 

BEGINNERS:

The most important trick of all is the ability to "flip" a flake from positive to negative.  Do your cut as shown above, you need only take an infinitesimally small amount (if any) of the flake out with this cut. ( I found out much later that this move can be greatly "x"aggerated as seen HERE.)

Make sure your cut's "loop" is on the right side & below  the flake's corner and that your cut ends left of the triangle's vertical line.  Release the mouse button and the flake will turn black.

Test to see if your flake has turned negative by drawing a shape into the black area.  It should turn white.  If it does NOT turn white then hit undo and do the move again until you are successful.

ADVANCED:

For NOVAS, PINWHEELS, GRIDS, etc...
Do the above cut over and over to achieve the movement of your flake. (This is what I call, "Throwing the flake into reverse.")

Be sure to watch the preview flake as you do your cuts to make sure your negative flip took and the animation is going in the direction it is supposed to!  Sometimes, if you're not paying attention, an occasional positive flip will happen and this will ruin the effect of "breathing" in Grids, or the spokes of a Pinwheel will "jerk", etc.

BEGINNERS:
This is easy!  You can turn any negative flake back to positive with this simple move.  Do your cut as shown above, start below and right of the triangle's right corner and take it to the other side, releasing the mouse button AFTER you have passed the triangle's left vertical line.  (This move worX both right to left OR left to right!)

ADVANCED:
No matter how many negative flips you have made in a row, doing this simple trick several times will bring you back to where you started.....SP's permitting, of course!