There is no real quick fix. It's something you need to work up to with practice and rote.
Flair should also be the last thing you work on; concentration on consistently creating a legible foundation. Random squiggles are the easiest to replicate and reproduce.
Also, once you have the fundamentals of handwriting down, the "flair" will come much more easily and naturally.
The first think you need to do, is learn how to properly hold a pen. Something like nine in ten people do so incorrectly. The common notion "just do what feels most comfortable" is a bit of a misnomer. There are simply muscles in your hand that work better at keeping the pen steady while controlling the accuracy of strokes than others. That's not to say you shouldn't ignore comfort all together, you'll just cramp and end up with carpal tunnel, but, like a good golf swing, and some point comfort has to cede to technique and form. I'm sure a simple Google will provide enough pictures and videos on pen holding.
The next step is to learn how to "write" with your shoulder and not your wrist. You simply have more control in your shoulder than you do your wrist. Learning how to write that way is the most important step in legible handwriting. There is a school of thought that says that isn't so true these days and is a remnant of the quill era. But in any calligraphy class (and even most adult art classes) it's the first thing they teach you.
The first thing they would have you do is write-out big "words" right in air in front of you making very broad strokes while concentrating on moving at your shoulder. Practice for a minute or so a few times a day for a few days.
Once you have the "feel" down you can move on to paper. A good way to start is not to focus on words or letters at all, but just simple symbols: X / \ O for example.
Write each one out repeatedly a few times and move on to the next while filling out a whole sheet of paper. Do this every day. Start with a fairly large size (maybe four or five lines) and gradually get smaller focusing on moving your whole arm and not just your hand.
Once you've done that for a few days you can start forming pseudo words. In your case, since you want to focus on your signature, go with your name. In my case (John) I would go with something like this: /OXO. This is to emulate the main pattern of strokes I would make in forming my name. Do this over and over for a few days until you can consistently and accurately form each group with the desired speed.
Then you can move onto real letters. Since the focus of this is your signature, just concentrate on the letters that make up your name.
Start with basic "cursive" forms--you know the ones they taught in second grade. In fact, swollow your pride and go find a book with the big dotted center line like you used back then. It may sound silly, but it will make a difference. You can also find a template online and print some off your self.
Then just write those letters in your name over and over and over.
Then once you got a grasp on the letters, star forming them into your name. (Again) concentrate on accuracy and constancy while writing form your shoulder. The more accurate and consistent you can write your name, the harder it will be for someone else to.
Once you can reproduce your name with near perfection, you can always stop. There's nothing wrong with having a signature in standard cursive. Heck, the a fore mentioned John Hancock pretty much did.
If you do want some style, then what I would do is search the interewbz for some handwriting type fonts. Or even just start with the ones in Word. Type out your name in a variety (even mix and match) in a fairly large font and print them out. Then try and emulate what you like.
Once you got that down, you can add some flair. To keep it simple, just exaggerate some letters or even the occasional loops and add some long tails.
But remember, the more flair the easier it is to forge because it's easy to replicated those exaggerated loops, tails and what not. They tend to become thing the cashiers, tellers, or whatever focus on--not how strait the vertical line on your "T" is.
