When you are running low on health, you click the sigil. After your first trip back to the citadel, you should always carry a tp scroll. After about level 6, you should always carry a sigil. The key to surviving is timing. Don't rush into enemy territory without backup, don't leave places for enemies to teleport in behind you. Never fight 2v3 or 1v3, only fight in even fights unless you know you can get a kill. When your hp gets low, use a sigil, then, the second an enemy uses an interrupt on you (such as penitence) teleport back to the crystal. If you play a slow character (rook), be extra cautious.

Well the trick with sigils is that they instantly cast, that surprise kills human players.

The trick with sigils

The more health you have, the more sigil works better.

Strike the right balance of high health and enough mana, and you will do fine.