Dark Transfer Papers need to be trimmed before applying to fabric.  The reason for this is because areas that are unprinted and untrimmed will appear white on the shirt.  There are a number of ways you can trim dark transfer papers.  You can use a pair of scissors, an exacto knife, a paper cutter (for square images), or a cutter/plotter with an optical eye (recommended method).

Using a cutter/plotter with an optical eye is the best method simply because it is extremely accurate and efficient.

How does the Print and Cut Feature Work On a Cutter/Plotter?
Most of the programs provided to you by the manufacturer to run your cutter will have a print and cut option (CutStudio for Roland GX 24, and GreatCuts for GCC cutters).  You simply turn on the print and cut feature either before or after (CutStudio is before and GreatCuts is after) you have finished laying out your graphic and cut lines.  Although you may have the print and cut option in your program, please ensure that your cutter has an optical eye.  Without an optical eye, the print and cut function will not work.

Once print and cut is activated in your software, you will see 3 or 4 registration marks on the corners of the transfer paper.  From this point, you can print your graphic along with the registration marks out on your printer.  This is how they would look:

After printing your graphic with the registration marks, you are ready to load your transfer paper into the cutter.  Once the paper is loaded, simply select cut in the software and the cutter will start cutting.  First the cutter will activate its optical eye system and you will see a red dot moving.  This is the cutter’s way of scanning for the registration marks.  Once it has read all 3 or 4 registration marks, it will cut the graphic out.   You can then weed the excess paper off the sheet.  Peel off the graphic or use tack to keep non-continuous parts together.  Then place on top of your fabric with image face up on your heat press.  Ensure you protect your graphic/image with a silicon or teflon sheet when pressing.

Here is a link to a video of this print and cut feature using CL Dark Premium

Why Do You Need the Print and Cut Registration Marks to Cut with Your Cutter?
The print and cut system is used so that the printer can communicate with the cutter via registration marks.  Simply put, the printer uses registration marks to tell the cutter where it has printed the graphic on the paper.  Without the registration marks and an optical eye, it is near impossible for the cutter to determine where the printer has printed the graphic on the paper.