The expression "charging" is a nod to tradition more than an accurate description of what is done to activate a sigil. In the old traditions of talismantic magic, once an amulet was created by whatever means, it had to be endowed with "energy" by some action of the magician. The world-view of the day determined what this "energy" was; in prehistory it would have contained a spirit, later a god or demon, eventually "life force" or "vibrations" provided the explanation
Making a sigil work is, in a way, the opposite of "charging". The idea is to open the gates of the deep mind and cause the sigil to be "absorbed" into it. The magician doesn't charge the sigil, the sigil charges the magician!
To do this, a state of gnosis must be achieved, and at that point, the sigil must be introduced to the deep mind. There are many variations on how this can be accomplished. The simplest is to hold the image of the sigil in front of your eyes and stare intently at it at the peak of gnosis and, to use Spare's description, "drink it in to the mind." It's very hard to use words to describe something so ineffable as this action.
I experience this as a state of high trance, of "not-thinking", but at the same time, with an obsessive concentration on the image of the sigil to the exclusion of all else. Not the meaning of the sigil, but simply the graphic image itself as pure abstract, unconnected to any meaning -- a picture and nothing more.