This is why in my former post, I tied the personal ward to the stationary one, in a sense defining the owner of the personal ward as belonging to the place where the stationary ward was based. This gives the mobile ward a broader foundation and makes it able to cope with more trouble than any normal all-purpose ward, provided of course, the stationary wards remain unbroken. Without the stationary wards to back it up or a strong will to keep it empowered, a personal ward is weaker.
Ideally I would say pick a spot that you consider safe, whether it's a random tree in the middle of the woods or a courthouse or your childhood home or whatever to you can justify as being incontrovertibly safe (and not likely to lose its safety) in your mind. The more you are tied to the place from personal experience, the better -so long as the symbolism is strong in your mind. Symbolism is the "circuitry" through which your intent flows. Don't tell anyone what or where this place is, just to be sure no one messes with it, and take some small bits from that site (tree bark, splinters, soil, paint chips, etc). Avoid returning to this place, else your enemies might make the connection if they know you are using this kind of ward. Use these in the manufacture of your charm as a kind of power source, ie a link to something larger than yourself and your own will.
In the wards I described before, I used vials for the personal amulets because they were most like the witches bottles. For something like this, you can use whatever "vessel" is convenient. So long as it can hold the "ingredients," it's good. So a vial, a lump of clay, a pouch, a tin, etc -so long as it's something you can carry on your person and not lose or forget. Something you can wear is ideal as you're less likely to lose it.
For a well-rounded protective stone as the focus of the charm, I prefer Sunstone. It has many benefits - bringing "blessings" to the user (calling to you good fortune to preempt any negative events), changing the negative for the positive (ensuring a positive outcome despite negative events), removing unhealthy psychological or spiritual attachments (ie preventing possession), dissipating fearfulness and stress (thus allowing you to keep such wasted energies for other things). However, different people have different personal energy. If you handle the stone and find you don't like it, you may have to do some research on your own.
Green, purple, white, and gold are the colors most associated with luck and protection. You may use these in construction of your charm, either paint or fabric or even stones (the sunstone would qualify as gold, or if you happen to be a metalsmith, using gold to make the entire talisman). It's not necessary that you use all of these colors, and it's very much a matter of personal preference. I mean if you make a charm that's so ghastly you can't bear to look at it, it won't do you much good. I also like to choose a shiny new penny for the luck symbolism and the copper content, copper being a very lucky metal.
There are many, many herbs which have protective energies and which you can use in the construction of such a talisman. To list them all would really be too much, but a quick google will give you dozens of sites from which to choose. Books are better simply because you won't always have internet access, they come with an index, and are more detailed than most internet entries. (Cunningham's and Beyerl's are probably the best in terms of availability, size, and content.) A good rule of thumb is to choose no more than seven plants (typically less and odd numbers at that) for any spell. Just remember, KISS- Keep It Simple Stupid (a favorite saying of a former teacher of mine). The spell is complicated enough without throwing more and more into it. Choose your ingredients based on what is available (kitchen spices for the win!), what is cheap (many useful "weeds" can be found in your backyard or nearby), and what has the broadest coverage in terms of the purpose of the spell. For instance, something that covers accidents is better than something that covers only fire; something that is protective and also adds power to other herbs is better than something which only protects against accidents, etc.
Finally, symbols. There are so many symbols that I have at least a dozen books on the subject. Again, stick to the basics. You don't need a ton of esoteric symbols to get the job done, unless to your mind that gives them added power. The Pentagram is one of the most protective symbols around, symbolic of the human body, the five elements, or the five senses, and therefore covering all the bases. The + is a simple sign which can be used to signify a crossroads or all directions, meaning help from all corners or protection from danger no matter its origin. The circle is about as simple as it gets in terms of protective signs, a wall to protect what is inside from everything that is outside.
And since I mentioned it in my previous post.... several years ago, I made everyone in my family protective amulets for their cars. It was not an amulet to prevent accidents, but rather to prevent injury. I had to focus on one or the other and opted for the more important goal of keeping the occupants of the car safe. So far as I know, my mother is the only one who took it seriously, but it worked in spectacular fashion.
She'd been hearing weird wheel noises for a while, but one day she was driving past a restaurant and turning a corner, her wheel popped off the car. The car more or less came to an immediate stop thanks to gravity. The wheel flew from the car, hit the large window of the restaurant, rebound back to the car, bounced off the car, and came to rest a few feet from the car. There were no injuries to anyone on the road, in the restaurant, or in the car.
Another time, she was on the road and the driver of a massive dump truck on the other side had a heart attack with his foot on the gas pedal. He plowed right past her and into the trees mere feet in front of her car. If it had struck her, she would have died instantly.
The charm itself:
3 pieces felt -purple, white, and green
red, black, white, purple, and blue embroidery floss
1 white ribbon
1 piece of yellow Jasper
1 new penny
equal parts Rosemary, comfrey, and moss
Cut the white felt into a six point star large enough to cover the penny. Cut the purple and green felt into large circles about the width of your palm. Sew the white star onto purple side with the penny underneath.
On the green felt, use the floss to sew the symbol of Saturn in black, Mars in red, Jupiter in blue, Mercury in purple, and the plus sign in white. These should be arranged more or less in the form of a pentagon with Saturn at the top point. Sew the two pieces of felt together inside out with a bit at the top open and then flip it right side out.
Using the ribbon, fasten the jasper to the outside of the charm, using the points as reference to make a pentagram. Trust me; it's just easier to do it this way. Then stuff it with the herbs and sew it shut our use the ribbon to make a drawstring to hang it from your rearview mirror. Anoint the stone with a few drops of myrrh oil and allow sun to shine on the stone as much as possible.