The Witcher 2 - Guide Henselt as he draws the magic runes Illustrated in Dethmold's grimoire

Here is a video illustrating how to do this exactly. You can refer to the following diagram or watch the video or just do it in the order below:

Henselt is already at the Pixie so from there go to:

1. Bread
2. Black candle
3. Goat
4. Scorched Tree
5. Raven
6. Sour Milk
7. Pixie



Basically you're doing the following pattern:

After that go down by the black candles and use your Igni sign and the whole thing will catch on fire.

***********To fight the monsters use the magic sign on yourself (Quen), it's the one that's the second from the last on the bottom I believe, it's the one that makes you glow blue and "shocks" anyone that tries to hit you (see pics below). Keep doing this sign on yourself for the duration of the battle. There's a lot of them to fight, after you beat a certain amount of them the cut-scene will ensue and Henselt will stab the bitch.