It depends on how you use the sigil. Sigil activation could pertain to sigils that function as talismans and engravings on amulets and or in direct contrast to sigils that are disposed of, and sent into the universe, the great abyss immediately after charging it.
Many activate there sigil after some meditation make sure the entire sigil is visible and gaze at it comfortably. Let your eyes become familiar with the symbol and meditate on the sigils power.
Sigils should be created by you and your specific intent and possess power from its creation and charging, but certain types of functions, when sigilised, require repeated activation. For example, a sigil of a deity on your body will function like an idol of the deity. By repeated chanting and meditating, you activate the power of the sigil and reinforce it.