To "enchant" an object is to fill it with energy for a specific purpose. While there are all sorts of "ookey-spooky," and dramatically ritualized methods of doing so, what it all basically boils down to is direct, programmed energy that has been gatherer and shaped by the will of the magician through deliberate visualization.  What happens in the mind is a microcosm of what happens in the macrocosm of the physical world.  This is what the phrase "As above, so below" means.  What happens in the microcosm of our minds can be directly reflected in the macrocosm of the world outside the mind.  

So to enchant an object, and there by change it into a talisman, all you need to do is concentrate on directing energy for a specific purpose and direct it into the object.  In the case of being "invisible" what you would want to do is visualize yourself walking along in a swirl of concealing mist (this mist represents the energy you are gathering).  In you mind, see people starting to look at the mist, only to have their gaze "slide" off, pulled to one side or drawn to something near you rather than focusing *on* you.  When you have built a clear image of this in your mind then visualize the ring and see the swirling mist of energy being drawn into and stored within the ring.  

When all of the mist has been drawn into the ring you're done... maybe.  At this point you can either "seal" the ring and be done with it, or you can add a "trigger" which will turn the power of the talisman on or off deliberately.  

With the former, the power of the talisman will be constantly active and the only way to stop it will be to take it off and place it in a bag or something that has been specifically charged to block the activity of the ring.

With the latter, you install an on/off switch, basically.  This can be either as set of spoken words of phrases (make sure it's something easy to remember, but not so common as to be used in every-day speech: "shields up" and "shields down" works well) or you can use a simple visualization (when you want it to activate, visualize the mist leaving the ring and surrounding you, and when you want it to deactivate visualize the mist being pulled back into the ring.)

Creating the trigger takes a little more work on your part, but is usually well worth it as you can wear the ring at all times without worry.  With the other method, you'll have to deliberately put it on and take it off, which increases the risk of loosing it... and few things are harder to find that an invisibility talisman!

Now as for a protection ritual, I have and example of a personal protection ritual if that's what you're actually looking for here

The ring we have been talking about in this example would be perfect to use as the “Personal item" that would be transformed into the talisman.

I would also like to point out that, while the ritual itself appears on the surface to be fairly elaborate, it could be easily adjusted.  The cleansing of the ring can be done simply by running it under clear water while visualizing all unwanted energies being washed down the drain, the runes can be drawn on a piece of white paper using ink of your astrological color (if possible write them in a circular pattern the put the ring inside the rune circle), and then just do the chants.

The key to this or any sort of magic is that you have a clear, solid "image" of your intent... what it is you want to have happen.   This is what it going to allow your subconscious to gather the energies you need and program them with your desired purpose.  If you are uncertain as to *exactly* what it is you intend to accomplish then your results will be uncertain.  You don't have to know the exact mechanics of "how" the task should be accomplished (in fact, it's usually best if you just set some basic parameters and let the universe take care of the details), but you do need to precisely know and understand the "what" and the "why."

Also, with an item of jewelry being used as the talisman, it is possible to have the item professionally engraved.  For our ring example, if the band is wide enough, you could take it and a straight-line rendition of the protection runes from the above ritual to a jeweler's and ask if they could engrave those symbols onto the ring for you (do this *before* you perform the ritual).

*A talisman is an object that has been specifically charged to perform a specific function.  Talismans are usually, but not always, man made.  An amulet, on the other hand, is an object of natural origin (i.e. crystals, stones, plants, *naturally shed* feathers, etc. which have certain "virtues" and magical properties as an inherent part of their existence.