To get curse prayers, open up your spellbook and teleport home to Edgeville using the teleport in the top left.
When you arrive at Edgeville, walk West until you see an NPC called the Lumbridge Sage South of the General Store.
When you see him, talk to him and select the "Could you take me to the Chaos Altar?" option.
This will teleport you to a small church just south of the wilderness ditch.
Walk into the church and pray at the Chaos Altar inside to the Southern side of it.
For in-game donators, getting Curse Prayers is much simpler.
All you have to do is open up the in-game donator tab inside the quest tab, and teleport to Donator Zone #2.
When you arrive, walk North-West to the Chaos Altar right next to a pair of magic trees.
Curse Prayer Book