What @MeganB explains in the videos is exactly what you need to know!
Using already existing symbols can be a very powerful addition to your sigil. It’s also a great way to start when you are stuck or have nothing in mind.
If you have an intention in mind, use a symbol that is associated to that. For example:
The spiral represents the universal pattern of growth and evolution. The spiral represents eternity and continuity. The spiral in nature appears frequently. It is a symbol that represents innocence, rebirth, and the eternal. The sacred spiral is also an energetic symbol, it represents energy. In fact, if you look at pure energy under a microscope you will see that energy forms spiral patterns. - Source
Well before Christianity, the fish symbol was known as “the Great Mother,” a pointed oval sign, the “vesica piscis” or Vessel of the Fish, which is also associated with the meaning behind the Triquetra. “Fish” and “womb” were synonymous terms in ancient Greek,“delphos.” Its link to fertility, birth, feminine sexuality and the natural force of women was acknowledged also by the Celts, as well as pagan cultures throughout northern Europe. - Source
The philosophers’ stone was the central symbol of the mystical terminology of alchemy, symbolizing perfection at its finest, enlightenment. It is a legendary alchemical substance capable of turning base metals such as mercury into gold.
Video with More Symbols
You can also use this website: Symbol Dictionary, as a starting point and then build your sigil from there once you know the approximate meaning of that symbol.
The meaning of a symbol or sigil is subjective and it can change depending on who reads it.