Noob question - before I get into online play - how do I use sigils?

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.

You need to use sigil if u have a lot of hp....at least minimum of 4k. Because sigil give u 50% of ur hp....if u have 4k u will gain 2k HP. Sigil will be usefull when u're running low in hp but u can still help ur team mate. And u can use sigil to save ur ass when u need to run away. Like splitShadow said....you need to always get a TP scroll on you. If u cant help ur team mate and ur about to die...use sigil and run back to the citadel or teleport to it.

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

Absolutely, a sigil is basically like a potion that restores half of your health instantly. The downside is that it only lasts twenty seconds. If you're asking yourself, why don't people use potions? Well that's because potions can be interrupted by spells like penitence (Oak) and deep freeze (torchbearer).

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.