Praise be to Allah.
Istijmaar means to cleanse the front or back passage of urine or stools using stones or something else in their place. One of the things that may take their place is tissue, but on the condition that one wipes no less than three times. And one should not use things that are forbidden to be used for istijmaar such as dung and bones, or things that have a kind of sanctity such as food etc.
It is permissible to do istijmaar with or without water. The scholars said, it is better to use both, because that is cleaner.