Basically, you keep them in your inventory while killing monsters that drop special ashes, such as demonic ashes or accursed ashes. The urn will automatically be filled with the ashes; when it is full, click on it to get some prayer experience.
Like all urns, after buying it you have to activate it with a rune first.
Runescape Accursed Urns give a bonus experience of 375 exp. To fill the urn you have to have the urn in your inventory and kill lesser or greater demons. The ashes will automatically go into the urn, and can be teleported when full.
Making pots, bowls, urns, and tablets.
The easiest 99 to get is Cooking, the Cooking exp per hour (on just Lobsters) is around 190k with the use of the best non members cooking urn. One urns exp bonus equates to 9 lobsters. Thus urns are a good investment. They reduce the amount of lobsters you need in total and from lvl 60-99 cooking, 1.1m of the exp will have been from urns! And as to date of June 2012, cooking is the fastest 99 to achieve.
To be accursed with something is to be afflicted or in possession. For example: If you had a disease or flu, you are said to be accursed.
Malekith the Accursed was created in 1984.
The Accursed Share was created in 1949.
Accursed Lands was created in 1996.
As many as you like (the limit for any item is about 2 billion). Once you start using it, you can't have more than 10 of those.
The Sand from the Urns was created in 1948.
Something which is accursed is something which has had a curse put upon it.
That dog has chewed up new carpeting, soiled my favorite quilt and left scratches on the front door. Now, I must get rid of this accursed animal.
Do not use 3rd Party software on RuneScape. It will get you banned. Do not use it.