It is ironically a difficult potion to make, requiring that the brewer follow the directions carefully, because making a mistake can have drastic consequences. Adding too much of the ingredients, for instance, will put the drinker of the potion into a deep — and possibly irreversible — sleep.
The directions for making the potion are very detailed. Ingredients must be added in the exact order and amounts specified, and the potion needs to be stirred 7 times, both clockwise and counter-clockwise. Before the addition of the final ingredient, hellebore, the temperature of the flames must be lowered, and the potion allowed to simmer for seven minutes.
If brewed correctly, the potion will emit a silvery vapour, but the possible characteristics of a failed batch seem to be endless (everything from green sparks, dark grey steam, a sulfurous odour, a cement-like consistency, to a dangerous instability that will cause it to explode and set one's robes alight).
This was the first potion that Severus Snape assigned to fifth year students in 1995. A lot of students did poorly, including Seamus Finnigan, Gregory Goyle, Neville Longbottom, and Ron Weasley. However, Professor Snape singled out Harry Potter, giving him alone a zero grade for the day's work by emptying out his cauldron for simply missing out the syrup of hellebore due to misreading the instructions.