"Cantrip" is an old Scots word for a piece of wilful trickery or mischief, and is another word for a witch's spell. A cantrip can also be a pre-formed spell, a piece of magic already set up in your mind, ready to call on at a moment's notice with a single word or gesture, or even a smell.
Let us say you wish to be able to form a shield around yourself to keep out negative signals, in a pub or supermarket for instance. A simple thing, just a mirror shield say, that makes you feel protected and secure. Before the situation arises, when you have space to work, decide on a key: a word, or a symbol you can trace in the air, with your finger or in your mind, or on a gesture - perhaps you would hold your fingers together in a certain way. Then cast the shield around yourself. Speak the word, make the gesture, trace the symbol first (if you can hold the gesture throughout, do so), and hold the thought of having done so in the front of your mind while you build the shield. Make the purpose of the exercise not just to build the shield, but to associate the act of building that shield, your own personal emergency shield, with the symbol or gesture or word you have chosen. You may wish to say something like "I build this shield (or perform some other act) now to bind it to this symbol/gesture/word. Once it is complete it will be bound in my mind." If you are using a word, make sure it is not a word you use frequently, or you may find yourself using the shield without actually intending to do so. It is also best to specify a time period for the binding, unless you are very sure of yourself, or else you may end up with assorted cantrips cluttering up your mind. Once you have finished, say "I bind this act to this symbol/gesture/word. It is bound in my mind (for however long)." You may like to repeat the binding a couple of times subsequently to be sure of it, particularly if you wish the cantrip to last a long time, but be sure you use the same actions, and be sure you are in a good state to be performing the spell - if you are tired when you make the binding, you will have a tired cantrip.
Whenever you need your basic mirror to protect you, you may use the cantrip. Speak the word - muttered under your breath is enough, no proclamations needed - make the gesture, trace the symbol. If the act is bound strongly enough, then you will need do no more. If you want to unbind a spell from a cantrip, use the key, and then dismiss the spell with the words "I release this action from this binding". Again, you might have to repeat a couple of times, making sure you have in your mind the concept of dissociation from the key.
Cantrips can be used for any small Working, such as shielding or deflecting or releasing a particular energy, and the keys can be used in combination. The more complex a Working, the harder it is to bind into a cantrip - but experiment with it. An aroma can be used for difficult bindings, as scent is one of the most evocative of experiences, and an aroma can be sensed throughout the working. Indeed scent is particularly good if you wish to make going into trance a cantrip, although it is best used in combination with a word or gesture to prevent accidents. Cantrips are particularly useful when one is experienced enough to know what works best for a particular situation but not practised enough to perform while under pressure. Cantrips can avoid the need to think of anything than the key to perform the act.